Geology topics

Brian G. Katz

Publications and source records attributed to Brian G. Katz.

At least 19 recordsLinked to original sources

USGS2015

The quality of our Nation's waters: Water quality in the Upper Floridan aquifer and overlying surficial aquifers, southeastern United States, 1993-2010

About 10 million people rely on groundwater from the Upper Floridan and surficial aquifers for drinking water. The Upper Floridan aquifer also is of primary importance to the region as a source of water for irrigation and as a source of crystal clear water that discharges to springs and streams providing recreational and tourist destinations and unique aquatic habitats. The reliance of the region on the Upper Floridan aquifer for drinking water and for the tourism and agricultural economies highlights the importance of long-term management to sustain the availability and quality of these resources.

Florida↗

USGS2015

The quality of our Nation's waters: Water quality in the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer, south-central United States, 1994-2008

About 8 million people rely on groundwater from the Mississippi embayment—Texas coastal uplands aquifer system for drinking water. The Mississippi River Valley alluvial aquifer also provides drinking water for domestic use in rural areas but is of primary importance to the region as a source of water for irrigation. Irrigation withdrawals from this aquifer are among the largest in the Nation and play a key role in the economy of the area, where annual crop sales total more than $7 billion. The reliance of the region on both aquifers for drinking water and irrigation highlights the importance of long-term management to sustain the availability and quality of these resources.

Alabama, Arkansas, Kentucky, Louisiana, Mississipp↗

USGS2014

Factors affecting public-supply well vulnerability in two karst aquifers

Karst aquifers occur in a range of climatic and geologic settings. Nonetheless, they are commonly characterized by their vulnerability to water-quality impairment. Two karst aquifers, the Edwards aquifer in south-central Texas and the Upper Floridan aquifer in western Florida, were investigated to assess factors that control the movement of contaminants to public-supply wells (PSWs). The geochemistry of samples from a selected PSW or wellfield in each aquifer was compared with that from nearby monitoring wells and regional PSWs. Geochemistry results were integrated with age tracers, flow modeling, and depth-dependent data to refine aquifer conceptual models and to identify factors that affect contaminant movement to PSWs. The oxic Edwards aquifer is vertically well mixed at the selected PSW/wellfield, although regionally the aquifer is geochemically variable downdip. The mostly anoxic Upper Floridan aquifer is affected by denitrification and also is geochemically variable with depth. In spite of considerable differences in geology and hydrogeology, the two aquifers are similarly vulnerable to anthropogenic contamination. Vulnerability in studied PSWs in both aquifers is strongly influenced by rapid karst flowpaths and the dominance of young (<10 years) groundwater. Vulnerability was demonstrated by the frequent detection of similar constituents of concern in both aquifers (nitrate, atrazine, deethylatrazine, tetrachloroethene, and chloroform). Specific consideration of water-quality protection efforts, well construction and placement, and aquifer response times to land-use changes and contaminant loading are discussed, with implications for karst groundwater management.

Ground Water↗

USGS2013

Estimating nitrate concentrations in groundwater at selected wells and springs in the surficial aquifer system and Upper Floridan aquifer, Dougherty Plain and Marianna Lowlands, Georgia, Florida, and Alabama, 2002-50

Groundwater from the surficial aquifer system and Upper Floridan aquifer in the Dougherty Plain and Marianna Lowlands in southwestern Georgia, northwestern Florida, and southeastern Alabama is affected by elevated nitrate concentrations as a result of the vulnerability of the aquifer, irrigation water-supply development, and intensive agricultural land use. The region relies primarily on groundwater from the Upper Floridan aquifer for drinking-water and irrigation supply. Elevated nitrate concentrations in drinking water are a concern because infants under 6 months of age who drink water containing nitrate concentrations above the U.S. Environmental Protection Agency maximum contaminant level of 10 milligrams per liter as nitrogen can become seriously ill with blue baby syndrome. In response to concerns about water quality in domestic wells and in springs in the lower Apalachicola–Chattahoochee–Flint River Basin, the Florida Department of Environmental Protection funded a study in cooperation with the U.S. Geological Survey to examine water quality in groundwater and springs that provide base flow to the Chipola River. A three-dimensional, steady-state, regional-scale groundwater-flow model and two local-scale models were used in conjunction with particle tracking to identify travel times and areas contributing recharge to six groundwater sites—three long-term monitor wells (CP-18A, CP-21A, and RF-41) and three springs (Jackson Blue Spring, Baltzell Springs Group, and Sandbag Spring) in the lower Apalachicola–Chattahoochee–Flint River Basin. Estimated nitrate input to groundwater at land surface, based on previous studies of nitrogen fertilizer sales and atmospheric nitrate deposition data, were used in the advective transport models for the period 2002 to 2050. Nitrate concentrations in groundwater samples collected from the six sites during 1993 to 2007 and groundwater age tracer data were used to calibrate the transport aspect of the simulations. Measured nitrate concentrations (as nitrogen) in wells and springs sampled during the study ranged from 0.37 to 12.73 milligrams per liter. Average apparent ages of groundwater calculated from measurements of chlorofluorocarbon, sulfur hexafluoride, and tritium from wells CP-18A, CP-21A,and RF-41 were about 23, 29, and 32 years, respectively. Average apparent ages of groundwater from Baltzell Springs Group, Sandbag Spring, and Jackson Blue Spring were about 16, 18, and 19 years, respectively. Simulated travel times of particles from the six selected sites ranged from less than 1 day to 511 years; both the minimum and maximum particle travel times were estimated for water from Jackson Blue Spring. Median simulated travel times of particles were about 30, 38, and 62 years for Jackson Blue Spring, Sandbag Spring, and Baltzell Springs Group, respectively. Study results indicated that travel times for approximately 50 percent of the particles from all spring sites were less than 50 years. The median simulated travel times of particles arriving at receptor wells CP-18A, CP-21A, and RF-41 were about 50, 35, and 36 years, respectively. All particle travel times were within the same order of magnitude as the tracer-derived average apparent ages for water, although slightly older than the measured ages. Travel time estimates were substantially greater than the measured age for groundwater reaching well CP-18A, as confirmed by the average apparent age of water determined from tracers. Local-scale particle-tracking models were used to predict nitrate concentrations in the three monitor wells and three springs from 2002 to 2050 for three nitrogen management scenarios: (1) fixed input of nitrate at the 2001 level, (2) reduction of nitrate inputs of 4 percent per year (from the previous year) from 2002 to 2050, and (3) elimination of nitrate input after 2001. Simulated nitrate concentrations in well CP-21A peaked at 7.82 milligrams per liter in 2030, and concentrations in background well RF-41 peaked at 1.10 milligrams per liter in 2020. The simulated particle travel times were longer than indicated by age dating analysis for groundwater in well CP-18A; to account for the poor calibration fit at this well, nitrate concentrations were shifted 21 years. With the shift, simulated nitrate concentrations in groundwater at CP-18A peaked at 13.76 milligrams per liter in 2026. For groundwater in Baltzell Springs Group, Jackson Blue Spring, and Sandbag Spring, simulated nitrate concentrations peaked at 3.77 milligrams per liter in 2006, 3.51 milligrams per liter in 2011, and 0.81 milligram per liter in 2018, respectively, under the three management scenarios. In management scenario 3 (elimination of nitrate input after 2001), simulated nitrate concentrations in Baltzell Springs Group decreased to less than background concentrations (0.10 milligram per liter) by 2033, and in Sandbag Spring concentrations decreased to less than background by 2041. Simulations using nitrate management scenarios 1 (fixed input of nitrate at 2001 levels) and 2 (reduction of 4.0 percent per year) indicate that nitrate concentrations in groundwater may remain above background concentrations through 2050 at all sites.

Alabama, Florida, Georgia↗

USGS2012

Occurrence and potential transport of selected pharmaceuticals and other organic wastewater compounds from wastewater-treatment plant influent and effluent to groundwater and canal systems in Miami-Dade County, Florida

An increased demand for fresh groundwater resources in South Florida has prompted Miami-Dade County to expand its water reclamation program and actively pursue reuse plans for aquifer recharge, irrigation, and wetland rehydration. The U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department (WASD) and the Miami-Dade Department of Environmental Resources Management (DERM), initiated a study in 2008 to assess the presence of selected pharmaceuticals and other organic wastewater compounds in the influent and effluent at three regional wastewater-treatment plants (WWTPs) operated by the WASD and at one WWTP operated by the City of Homestead, Florida (HSWWTP).

Florida↗

USGS2011

Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system

Groundwater chemistry and tracer-based age data were used to assess contaminant movement and geochemical processes in the middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system. Water samples were collected from 30 drinking-water wells (mostly domestic and public supply) and analyzed for nutrients, major ions, pesticides, volatile organic compounds (VOCs), and transient age tracers (chlorofluorocarbons, tritium and helium-3, and sulfur hexafluoride). Redox conditions are highly variable throughout the MCA. However, mostly oxic groundwater with low dissolved solids is more vulnerable to nitrate contamination in the outcrop areas east of the Mississippi River in Mississippi and west Tennessee than in mostly anoxic groundwater in downgradient areas in western parts of the study area. Groundwater in the outcrop area was relatively young (apparent age of less than 40 years) with significantly ( p < 0.05) higher dissolved oxygen and nitrate–N concentrations and higher detections of pesticides and VOCs compared to water samples from wells in downgradient areas. Oxygen reduction and denitrification rates were low compared to other aquifers in the United States (zero order rate constants for oxygen reduction and denitrification were 4.7 and 5–10 μmol/L/year, respectively). Elevated concentrations of nitrate–N, and detections of pesticides and VOCs in some deep public supply wells (>50 m depth) indicated contaminant movement from shallow parts of the aquifer into deeper oxic zones. Given the persistence of nitrate in young oxic groundwater that was recharged several decades ago, and the lack of a confining unit, the downward movement of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing older oxic water.

Claiborne Aquifer;Mississippi Embayment↗

USGS2010

Chemical evolution of groundwater near a sinkhole lake, northern Florida: 2. Chemical patterns, mass-transfer modeling, and rates of chemical reactions

Chemical patterns along evolutionary groundwater flow paths in silicate and carbonate aquifers were interpreted using solute tracers, carbon and sulfur isotopes, and mass balance reaction modeling for a complex hydrologic system involving groundwater inflow to and outflow from a sinkhole lake in northern Florida. Rates of dominant reactions along defined flow paths were estimated from modeled mass transfer and ages obtained from CFC-modeled recharge dates. Groundwater upgradient from Lake Barco remains oxic as it moves downward, reacting with silicate minerals in a system open to carbon dioxide (CO 2 ), producing only small increases in dissolved species. Beneath and downgradient of Lake Barco the oxic groundwater mixes with lake water leakage in a highly reducing, silicate-carbonate mineral environment. A mixing model, developed for anoxic groundwater downgradient from the lake, accounted for the observed chemical and isotopic composition by combining different proportions of lake water leakage and infiltrating meteoric water. The evolution of major ion chemistry and the 13 C isotopic composition of dissolved carbon species in groundwater downgradient from the lake can be explained by the aerobic oxidation of organic matter in the lake, anaerobic microbial oxidation of organic carbon, and incongruent dissolution of smectite minerals to kaolinite. The dominant process for the generation of methane was by the CO 2 reduction pathway based on the isotopic composition of hydrogen (δ 2 H(CH 4 ) = −186 to −234‰) and carbon (δ 13 C(CH 4 ) = −65.7 to −72.3‰). Rates of microbial metabolism of organic matter, estimated from the mass transfer reaction models, ranged from 0.0047 to 0.039 mmol L −1 yr −1 for groundwater downgradient from the lake.

Water Resources Research↗

USGS2010

Chemical evolution of groundwater near a sinkhole lake, northern Florida: 1. Flow patterns, age of groundwater, and influence of lakewater leakage

Leakage from sinkhole lakes significantly influences recharge to the Upper Floridan aquifer in poorly confined sediments in northern Florida. Environmental isotopes (oxygen 18, deuterium, and tritium), chlorofluorocarbons (CFCs: CFC-11, CCl 3 F; CFC-12, CCl 2 F 2 ; and CFC-113, C 2 Cl 3 F 3 ), and solute tracers were used to investigate groundwater flow patterns near Lake Barco, a seepage lake in a mantled karst setting in northern Florida. Stable isotope data indicated that the groundwater downgradient from the lake contained 11–67% lake water leakage, with a limit of detection of lake water in groundwater of 4.3%. The mixing fractions of lake water leakage, which passed through organic-rich sediments in the lake bottom, were directly proportional to the observed methane concentrations and increased with depth in the groundwater flow system. In aerobic groundwater upgradient from Lake Barco, CFC-modeled recharge dates ranged from 1987 near the water table to the mid 1970s for water collected at a depth of 30 m below the water table. CFC-modeled recharge dates (based on CFC-12) for anaerobic groundwater downgradient from the lake ranged from the late 1950s to the mid 1970s and were consistent with tritium data. CFC-modeled recharge dates based on CFC-11 indicated preferential microbial degradation in anoxic waters. Vertical hydraulic conductivities, calculated using CFC-12 modeled recharge dates and Darcy's law, were 0.17, 0.033, and 0.019 m/d for the surficial aquifer, intermediate confining unit, and lake sediments, respectively. These conductivities agreed closely with those used in the calibration of a three-dimensional groundwater flow model for transient and steady state flow conditions.

Florida↗

USGS2010

Occurrence of Organic Compounds in Source and Finished Samples from Seven Drinking-Water Treatment Facilities in Miami-Dade County, Florida, 2008

The U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department, conducted a reconnaissance study in 2008 to determine the occurrence of 228 organic compounds in raw, source (untreated) and finished (treated) drinking water at seven municipal water-treatment facilities in Miami-Dade County. Results of this sampling study showed that 25 (about 11 percent) of the 228 organic compounds were detected in at least one source water sample and 22 (about 10 percent) were detected in at least one finished water sample. The concentrations of organic compounds in source water samples were less than or equal to 0.2 (u or mu)g/L (micrograms per liter). The concentrations of organic compounds in finished water samples were generally less than or equal to 0.5 (u or mu)g/L, with the exception of bromoform (a possible disinfection byproduct) at estimated concentrations ranging from 0.7 to 2.8 (u or mu)g/L and diethyl phthalate (a plasticizer compound) at 2 (u or mu)g/L.

Florida↗

USGS2010

Nitrate-N movement in groundwater from the land application of treated municipal wastewater and other sources in the Wakulla Springs springshed, Leon and Wakulla Counties, Florida, 1966-2018

The City of Tallahassee began a pilot study in 1966 at the Southwest Farm sprayfield to determine whether disposal of treated municipal wastewater using center pivot irrigation techniques to uptake nitrate-nitrogen (nitrate-N) is feasible. Based on the early success of this project, a new, larger Southeast Farm sprayfield was opened in November 1980. However, a recent 2002 study indicated that nitrate-N from these operations may be moving through the Upper Floridan aquifer to Wakulla Springs, thus causing nitrate-N concentrations to increase in the spring water. The increase in nitrate-N combined with the generally clear spring water and abundant sunshine may be encouraging invasive plant species growth. Determining the link between the nitrate-N application at the sprayfields and increased nitrate-N levels is complicated because there are other sources of nitrate-N in the Wakulla Springs springshed, including atmospheric deposition, onsite sewage disposal systems, disposal of biosolids by land spreading, creeks discharging into sinks, domestic fertilizer application, and livestock wastes. Groundwater flow and fate and transport modeling were conducted to simulate the effect of all of the nitrate-N sources on Wakulla Springs from January 1, 1966, through December 31, 2018. The total simulated nitrate-N load to Wakulla Springs in 1967 was a relatively modest 69,000 kilograms per year (kg/yr). The major sources of the nitrate-N load in 1967 were determined to be: 1. Inflow to the study area across the lateral model boundaries at 31,000 kg/yr (45 percent), 2. Biosolids disposal by land spreading at 14,000 kg/yr (20 percent), 3. Creeks discharging into sinks at 7,800 kg/yr (11 percent), and 4. The Southwest Farm sprayfield at 4,500 kg/yr (7 percent). The total simulated nitrate-N load to Wakulla Springs in 1987 had increased dramatically to 297,000 kg/yr. The major sources of nitrate-N load in 1987 were determined to be: 1. The Southeast Farm sprayfield at 186,000 kg/yr (63 percent), 2. Biosolids at 37,000 kg/yr (12 percent), and 3. Inflow to the study area across the lateral model boundaries at 36,000 at kg/yr (12 percent). All of the other sources were 5 percent or less. The Wakulla Springs discharge can change rapidly, even during periods of little or no rainfall. This rapid change is probably the result of Wakulla Springs intermittently capturing groundwater that has been going to the Spring Creek Springs Group. This spring group is located in a marine estuary and is affected by tidally influenced saltwater intrusion. Two modeling scenarios were simulated and results are presented for 2007 and 2018 in an effort to bracket the range of possible current and future changes in the flow of Wakulla Springs. In scenario 1, it was assumed that Wakulla Springs was not capturing Spring Creek Springs Group flow. In scenario 2, it was assumed that Wakulla Springs was capturing Spring Creek Springs Group flow. Under the assumptions of scenario 1, the total simulated nitrate-N load to Wakulla Springs in 2007 was 207,200 kg/yr. The major sources of nitrate-N load were determined to be: 1. The Southeast Farm sprayfield at 111,000 kg/yr 53 percent), 2. Inflow to the study area across the lateral model boundaries at 44,000 at kg/yr (21 percent), and 3. Onsite sewage disposal systems at 24,000 kg/yr (12 percent). All of the other sources contributed 6 percent or less. Under the assumptions of scenario 2, the total simulated nitrate-N load to Wakulla Springs was 294,000 kg/yr. The major sources of nitrate-N load were determined to be: 1. The Southeast Farm sprayfield at 111,000 kg/yr (38 percent), 2. Onsite sewage disposal systems at 56,000 kg/yr (19 percent), 3. Inflow to the study area across the lateral model boundaries at 52,000 at kg/yr (18 percent), and 4. Creeks discharging into sinks at 31,000 kg/yr (11 percent). All of the other sources contributed 8 percent or less. The nitrate-N loads to Wakulla Springs from the Southeast Farm sprayfield for scenarios 1 and 2 were both 111,000 kg/yr. These amounts were the same because most of the water from the Southeast Farm sprayfield went into Wakulla Springs in both simulations. In contrast, the nitrate-N loads from onsite sewage disposal systems for scenarios 1 and 2 were 24,000 kg/yr and 56,000 kg/yr, respectively. The additional water captured by Wakulla Springs in scenario 2 came from an area that had a high density of residential and commercial sites using onsite sewage disposal systems Under the assumptions of scenario 1, the total simulated nitrate-N load to Wakulla Springs in 2018 will be 156,000 kg/yr. The major sources of nitrate-N load for scenario 1 are anticipated to be: 1. Inflow to the study area across the lateral model boundaries at 48,000 at kg/yr (31 percent), 2. The Southeast Farm sprayfield at 42,000 kg/yr (27 percent), 3. Onsite sewage disposal systems at 32,000 kg/yr (21 percent), and 4. Fertilizer at 17,000 kg/yr (11 percent). All of the other sources will contribute 5 percent or less. Under the assumptions of scenario 2, the total simulated nitrate-N load to Wakulla Springs in 2018 will be 266,000 kg/yr. The major sources of nitrate-N load for scenario 2 are anticipated to be: 1. Onsite sewage disposal systems at 80,000 kg/yr (30 percent), 2. Inflow to the study area across the lateral model boundaries at 57,000 at kg/yr (21 percent), 3. The Southeast Farm sprayfield at 43,000 kg/yr (16 percent), 4. Creeks discharging into sinks at 31,000 kg/yr (12 percent), and 5. Fertilizer at 32,000 kg/yr (12 percent). All of the other sources will contribute 6 percent or less. The simulated nitrate-N load from the Southeast Farm sprayfield to Wakulla Springs during 2007 through 2018 decreases from 111,000 kg/yr to 42,000 kg/yr in scenario 1 and decreases from 111,000 kg/yr to 43,000 kg/yr in scenario 2. Both scenarios show these decreases because of the simulated planned reduction in the concentration of nitrate-N in the wastewater used for irrigation from approximately 12 milligrams per liter (mg/L) in 2007 to 3 mg/L in 2018. In contrast, the simulated nitrate-N load from onsite sewage disposal systems to Wakulla Springs from 2007 through 2018 increases from 24,000 kg/yr to 32,000 kg/yr in scenario 1, and increases from 56,000 kg/yr to 80,000 kg/yr in scenario 2. Both scenarios show increases respective to the increases in population and residential and commercial sites using onsite sewage disposal systems. In addition, the simulated nitrate-N load to Wakulla Springs from 2007 through 2018 from inflow to the study area across the lateral model boundaries increases from 44,000 kg/yr to 48,000 kg/yr in scenario 1, and increases from 54,000 kg/yr to 57,000 kg/yr in scenario 2. Both scenarios show increases due to increasing nitrate-N levels upgradient in Leon County.

Scientific Investigations Report↗

USGS2009

Depth-dependent sampling to identify short-circuit pathways to public-supply wells in multiple aquifer settings in the United States

Depth-dependent water-quality and borehole flow data were used to determine where and how contamination enters public-supply wells (PSWs) at study sites in different principal aquifers of the United States. At each of three study sites, depth-dependent samples and wellbore flow data were collected from multiple depths in selected PSWs under pumping conditions. The chemistry of these depth-dependent samples, along with samples of the surface discharge from the PSWs, was compared to that of adjacent nested monitoring wells. The results of depth-dependent analyses from sites in Modesto (California), York (Nebraska), and Tampa (Florida) are summarized and compared. Although the exact mechanisms for transport of contaminants to the PSWs varied among these hydrogeologic settings, in all three settings the presence of wells or boreholes or natural preferential flow paths allowed water and contaminants to bypass substantial portions of the aquifer and to reach PSWs or depths in the aquifer more quickly than would have occurred in the absence of these short-circuiting flow paths. The chemistry and flow data from multiple depths was essential to developing an understanding of the dominant flow paths of contaminants to PSW in all three settings. This knowledge contributes to developing effective strategies for monitoring and protection.

Hydrogeology Journal↗

USGS2009

Relations between sinkhole density and anthropogenic contaminants in selected carbonate aquifers in the eastern United States

The relation between sinkhole density and water quality was investigated in seven selected carbonate aquifers in the eastern United States. Sinkhole density for these aquifers was grouped into high (>25 sinkholes/100 km 2 ), medium (1–25 sinkholes/100 km 2 ), or low (<1 sinkhole/100 km 2 ) categories using a geographical information system that included four independent databases covering parts of Alabama, Florida, Missouri, Pennsylvania, and Tennessee. Field measurements and concentrations of major ions, nitrate, and selected pesticides in samples from 451 wells and 70 springs were included in the water-quality database. Data were collected as a part of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Areas with high and medium sinkhole density had the greatest well depths and depths to water, the lowest concentrations of total dissolved solids and bicarbonate, the highest concentrations of dissolved oxygen, and the lowest partial pressure of CO 2 compared to areas with low sinkhole density. These chemical indicators are consistent conceptually with a conduit-flow-dominated system in areas with a high density of sinkholes and a diffuse-flow-dominated system in areas with a low density of sinkholes. Higher cave density and spring discharge in Pennsylvania also support the concept that the high sinkhole density areas are dominated by conduit-flow systems. Concentrations of nitrate-N were significantly higher ( p < 0.05) in areas with high and medium sinkhole density than in low sinkhole-density areas; when accounting for the variations in land use near the sampling sites, the high sinkhole-density area still had higher concentrations of nitrate-N than the low sinkhole-density area. Detection frequencies of atrazine, simazine, metolachlor, prometon, and the atrazine degradate deethylatrazine indicated a pattern similar to nitrate; highest pesticide detections were associated with high sinkhole-density areas. These patterns generally persisted when analyzing the detection frequency by land-use groups, particularly for agricultural land-use areas where pesticide use would be expected to be higher and more uniform areally compared to urban and forested areas. Although areas with agricultural land use and a high sinkhole density were most vulnerable (median nitrate-N concentration was 3.7 mg/L, 11% of samples exceeded 10 mg/L, and had the highest frequencies of pesticide detection), areas with agricultural land use and low sinkhole density still were vulnerable to contamination (median nitrate-N concentration was 1.5 mg/L, 8% of samples exceeded 10 mg/L, and had some of the highest frequencies of detections of pesticides). This may be due in part to incomplete or missing data regarding karst features (such as buried sinkholes, low-permeability material in bottom of sinkholes) that do not show up at the scales used for regional mapping and to inconsistent methods among states in karst feature delineation.

Missouri↗

USGS2009

Assessing the vulnerability of public-supply wells to contamination: Floridan aquifer system near Tampa, Florida

This fact sheet highlights findings from the vulnerability study of a public-supply well in Temple Terrace, Florida, northeast of Tampa. The well selected for study typically produces water at the rate of 700 gallons per minute from the Upper Floridan aquifer. Water samples were collected at the public-supply well and at monitoring wells installed in or near the simulated zone of contribution to the supply well. Samples of untreated water from the public-supply wellhead contained the undesirable constituents nitrate, arsenic, uranium, radon-222, volatile organic compounds (VOCs), and pesticides, although all were detected at concentrations less than established drinking-water standards, where such standards exist. Overall, study findings point to four primary factors that affect the movement and fate of contaminants and the vulnerability of the public-supply well in Temple Terrace: (1) groundwater age (how long ago water entered, or recharged, the aquifer); (2) short-circuiting of contaminated water through sinkholes; (3) natural geochemical processes within the aquifer; and (4) pumping stress. Although the public-supply well is completed in the Upper Floridan aquifer, it produces water with concentrations of nitrate, VOCs, and the natural contaminant radon that are intermediate between the typical composition of water from the Upper Floridan aquifer and that of the overlying surficial aquifer system. Mixing calculations show that the water produced by the public-supply well could consist of upwards of 50 percent water from the surficial aquifer system mixed with water from the Upper Floridan aquifer. Anthropogenically affected water from the surficial aquifer system travels rapidly to depth through sinkholes that must be directly connected to the cavernous zone intersected by the public-supply well (and several other production wells in the region). Such solution features serve as fast pathways to the well and circumvent the natural attenuation of nitrate and radon that occurs when water from the surficial aquifer flows downward through the confining unit and then through the Upper Floridan aquifer matrix. Roughly 50 percent of the simulated flow to the public-supply well consists of water less than about 10 years old, thus making the well vulnerable to contamination from human activities. Sampling at various depths in the public-supply well during pumping and nonpumping conditions showed that water entering the well from the cavernous zone had much higher arsenic concentrations during pumping conditions (18.9 ug/L) than during nonpumping conditions (4.2 ug/L). This implies that movement of arsenic to the public-supply well from the cavernous zone is enhanced by pumping. One possible explanation is that pumping increases the movement of water with elevated dissolved oxygen content through the cavernous zone, which causes dissolution of arsenic associated with pyrite. All public-supply wells in the area may not have the same level of vulnerability as the well studied - many of the public-supply wells in the region have lower pumping rates and longer open intervals that may draw in a larger proportion of old water that predates anthropogenic influences. Determining the similarity of water produced by various public-supply wells in the region to that of the surficial aquifer system is one measure of well vulnerability that could be used to prioritize monitoring and land-use planning efforts to protect the most vulnerable wells.

Florida↗

USGS2009

Simulations of Groundwater Flow and Particle Tracking Analysis in the Area Contributing Recharge to a Public-Supply Well near Tampa, Florida, 2002-05

Shallow ground water in the north-central Tampa Bay region, Florida, is affected by elevated nitrate concentrations, the presence of volatile organic compounds, and pesticides as a result of groundwater development and intensive urban land use. The region relies primarily on groundwater for drinking-water supplies. Sustainability of groundwater quality for public supply requires monitoring and understanding of the mechanisms controlling the vulnerability of public-supply wells to contamination. A single public-supply well was selected for intensive study based on the need to evaluate the dominant processes affecting the vulnerability of public-supply wells in the Upper Floridan aquifer in the City of Temple Terrace near Tampa, Florida, and the presence of a variety of chemical constituents in water from the well. A network of 29 monitoring wells was installed, and water and sediment samples were collected within the area contributing recharge to the selected public-supply well to support a detailed analysis of physical and chemical conditions and processes affecting the water chemistry in the well. A three-dimensional, steady-state groundwater flow model was developed to evaluate the age of groundwater reaching the well and to test hypotheses on the vulnerability of the well to nonpoint source input of nitrate. Particle tracking data were used to calculate environmental tracer concentrations of tritium and sulfur hexafluoride and to calibrate traveltimes and compute flow paths and advective travel times in the model area. The traveltime of particles reaching the selected public-supply well ranged from less than 1 day to 127.0 years, with a median of 13.1 years; nearly 45 percent of the simulated particle ages were less than about 10 years. Nitrate concentrations, derived primarily from residential/commercial fertilizer use and atmospheric deposition, were highest (2.4 and 6.11 milligrams per liter as nitrogen, median and maximum, respectively) in shallow groundwater from the surficial aquifer system and lowest (less than the detection level of 0.06 milligram per liter) in the deeper Upper Floridan aquifer. Denitrification occurred near the interface of the surficial aquifer system and the underlying intermediate confining unit, within the intermediate confining unit, and within the Upper Floridan aquifer because of reducing conditions in this part of the flow system. However, simulations indicate that the rapid movement of water from the surficial aquifer system to the selected public-supply well through karst features (sinkholes) and conduit layers that bypass the denitrifying zones (short-circuits), coupled with high pumping rates, allow nitrate to reach the selected public-supply well in concentrations that resemble those of the overlying surficial aquifer system. Water from the surficial aquifer system with elevated concentrations of nitrate and low concentrations of some volatile organic compounds and pesticides is expected to continue moving into the selected public-supply well, because calculated flux-weighted concentrations indicate the proportion of young affected water contributing to the well is likely to remain relatively stable over time. The calculated nitrate concentration in the selected public-supply well indicates a lag of 1 to 10 years between peak concentrations of nonpoint source contaminants in recharge and appearance in the well.

Scientific Investigations Report↗

USGS2009

Factors affecting water quality in selected carbonate aquifers in the United States, 1993-2005

Carbonate aquifers are an important source of water in the United States; however, these aquifers can be particularly susceptible to contamination from the land surface. The U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program collected samples from wells and springs in 12 carbonate aquifers across the country during 1993–2005; water-quality results for 1,042 samples were available to assess the factors affecting ground-water quality. These aquifers represent a wide range of climate, land-use types, degrees of confinement, and other characteristics that were compared and evaluated to assess the effect of those factors on water quality. Differences and similarities among the aquifers were also identified. Samples were analyzed for major ions, radon, nutrients, 47 pesticides, and 54 volatile organic compounds (VOCs). Geochemical analysis helped to identify dominant processes that may contribute to the differences in aquifer susceptibility to anthropogenic contamination. Differences in concentrations of dissolved oxygen and dissolved organic carbon and in ground-water age were directly related to the occurrence of anthropogenic contaminants. Other geochemical indicators, such as mineral saturation indexes and calcium-magnesium molar ratio, were used to infer residence time, an indirect indicator of potential for anthropogenic contamination. Radon exceeded the U.S. Environmental Protection Agency proposed Maximum Contaminant Level (MCL) of 300 picocuries per liter in 423 of 735 wells sampled, of which 309 were drinking-water wells. In general, land use, oxidation-reduction (redox) status, and degree of aquifer confinement were the most important factors affecting the occurrence of anthropogenic contaminants. Although none of these factors individually accounts for all the variation in water quality among the aquifers, a combination of these characteristics accounts for the majority of the variation. Unconfined carbonate aquifers that had high percentages of urban or agricultural land, or a combination of both, had higher concentrations and higher frequency of detections for most of the anthropogenic contaminants than areas with other combinations of land use and degree of aquifer confinement. Redox status is an indicator of more recently recharged water and affects the fate of some contaminants. Median concentrations of nitrate were highest in the Valley and Ridge and Piedmont aquifers and lowest in the Biscayne and Silurian-Devonian/Upper carbonate aquifers. Nitrate concentrations were significantly higher in unconfined aquifers than in confined aquifers and semiconfined/mixed confined aquifers (wells in aquifers with breached confining layers or wells open to both a confined and an unconfined aquifer). Water recharged after 1953 had significantly higher concentrations of nitrate than water recharged prior to 1953. Redox status was also a key factor affecting nitrate concentrations; in recently recharged waters, samples in oxic waters had significantly higher concentrations of nitrate than anoxic waters, regardless of land use in the area around the well. Samples from 54 wells (5 percent) exceeded the U.S. Environmental Protection Agency MCL of 10 mg/L for nitrate in drinking water. Most of the samples exceeding the drinking-water standard (52 samples, or 5 percent) were in domestic supply wells in agricultural areas. The Piedmont and Valley and Ridge aquifers had the largest number of samples (45) exceeding the MCL; in the remaining aquifers only 9 samples had concentrations of nitrate that exceeded the MCL (about 1 percent). None of the water recharged prior to 1953 and only a single sample from a confined aquifer had nitrate concentrations that exceeded 10 mg/L as N. Wells were sampled for a minimum of 47 pesticides. Detection frequencies and comparisons varied depending on the assessment level used. At least 1 of the 47 pesticides was detected at 510 (50 percent) of the 1,027 sites where pesticide data were available using the ‘all detections’ assessment level—that is, including any quantified detection as well as any estimated values where the compound was definitively detected. Multiple pesticides were frequently detected in a sample of water from a site; 34 percent of the samples had two to five pesticides detected in the same sample, and 4 percent of the samples had six or more pesticides detected. Dieldrin was detected at 20 sites, 9 of which were from either domestic or public supply wells, at a concentration above the Health-Based Screening Level (HBSL) of 0.002 µg/L. Diazinon was detected at a concentration greater than the HBSL of 1 µg/L at a single site, which was also a domestic supply well. These are the only samples where a pesticide exceeded a human-health benchmark. The most frequently occurring pesticide compounds were four herbicides—atrazine, simazine, metolachlor, and prometon—and deethylatrazine, a degradate of atrazine. These pesticides typically were detected at concentrations that were less than 10 percent of a human-health benchmark. Of the four frequently occurring pesticides, only samples for atrazine (3 percent) and simazine (0.1 percent) had concentrations that exceeded 10 percent of the human-health benchmark; most of these cases were in agricultural areas. It is important to note, however, that the most frequently occurring pesticide degradate compound—deethylatrazine—has no human-health benchmark. Using a common assessment level of 0.01 µg/L, four of the aquifers—Biscayne, Mississippian, Piedmont, and Valley and Ridge—had at least one of these five compounds detected in more than 30 percent of the wells sampled. These four aquifers, along with the Ordovician, Ozark Plateaus, and Prairie du Chien aquifers were the aquifers or aquifer systems that had concentrations of pesticides that exceeded 10 percent of a human-health benchmark. Water recharged after 1953 had a significantly higher percentage of detections of pesticides than water recharged before 1953, and water from unconfined aquifers had a significantly higher percentage of detections of pesticides than water from confined or semiconfined/mixed confined aquifers. Water from sites in unconfined aquifers, where land use was agricultural or urban, accounted for the vast majority of detections of pesticides. Dissolved oxygen concentration was positively related to pesticide occurrence, which likely reflects the positive association between dissolved oxygen concentration and recently recharged water. Water samples were collected for analysis of VOCs at 793 sites—154 samples were analyzed for 54 VOCs from 1993 through 1995 and 639 samples were analyzed for 86 VOCs from 1996 through 2005. Twenty percent of samples contained one or more VOCs at concentrations greater than or equal to 0.2 µg/L (159 of 793 samples). The aquifers with the highest percentage of samples containing one or more VOCs were the Castle Hayne (about 41 percent of samples) and Biscayne aquifers (34 percent). The most frequently detected VOCs were chloroform, tetrahydrofuran, tetrachloroethene (PCE), toluene, acetone, ethylmethylketone, methyl tert-butyl ether (MTBE), and trichloroethene (TCE). Low-level concentrations of VOCs occurred in a much larger percentage of a subset of the data (the 639 samples analyzed using a low-level analytical method). In these samples, 69 percent of the 639 samples contained 1 or more VOCs, indicating the vulnerability of the carbonate aquifers to low-level VOC contamination. Four VOCs were detected at concentrations exceeding their respective MCLs in five samples, all of which were from drinking-water wells. Vinyl chloride concentrations exceeded the MCL of 2 µg/L in two samples from urban areas in the unconfined Biscayne aquifer. PCE, TCE, and 1,2-dichloropropane each had one sample with a concentration greater than their MCLs of 5 µg/L; these samples were from agricultural and urban areas in the unconfined Mississippian aquifer. Water quality in the 12 carbonate aquifers was highly variable. Most of the samples met drinking-water standards. The occurrence of anthropogenic contaminants was related to contaminant sources but also was affected by degree of aquifer confinement, ground-water age, and redox status. Areas with higher amounts of agricultural or urban land in unconfined aquifers were the most likely to have elevated concentrations of anthropogenic contaminants.

Scientific Investigations Report↗

USGS2009

Hydrology, Water Quality, and Aquatic Communities of Selected Springs in the St. Johns River Water Management District, Florida

Hydrologic, physicochemical, and aquatic community data were collected and compiled by the U.S. Geological Survey for selected springs within the St. Johns River Water Management District from January 2004 to October 2007. Nine springs were included in this study: Alexander, Apopka, Bugg, De Leon, Gemini, Green, Rock, Silver Glen, and Wekiwa. Urban lands increased in Alexander, Apopka, De Leon, Gemini, Green, and Wekiwa springsheds between 1973 and 2004, accompanied by a loss of forested and/or agricultural lands in most springsheds. Forested cover increased and open surface waters and wetlands decreased in the Bugg and Rock springsheds. Although rainfall did not change significantly over time in each springshed, spring discharge decreased significantly in De Leon, Fern Hammock, Rock, Silver, and Wekiwa Springs. Nitrate concentrations increased significantly with time in Apopka, Fern Hammock, Gemini Springs run, and Juniper Springs, and decreased significantly in Alexander Spring, Bugg Spring run, Rock Springs, and Wekiwa Springs. Phosphorus increased significantly with time in Juniper Springs and decreased significantly in Apopka, De Leon, Rock, Silver Glen, and Wekiwa Springs. Benthic macroinvertebrate communities ranged from relatively low diversity assemblages (Green Spring) to assemblages with high taxonomic richness, diversity, and dominance (Rock and De Leon Springs). Shannon-Wiener diversity index averages among samples pooled by spring were lowest for Apopka Spring and greatest for Rock, Bugg, and Silver Glen Springs. Mean Stream Condition Index for pooled samples per spring was lowest for De Leon and Gemini Springs and highest for Rock and Wekiwa Springs. Mean percentages of very tolerant taxa were lowest for Alexander Spring and highest for Bugg and Green Springs. Fish community richness was lowest for Green Spring, and greatest for Alexander Spring run and Silver Glen Springs. Forty five fish species representing 35 genera and 23 families were collected or observed from all springs in this study. Samples were dominated by centrarchids, cyprinids, fundulids, atherinopsids, and poeciliids.

Scientific Investigations Report↗

USGS2007

Hydrogeologic investigation, water chemistry analysis, and model delineation of contributing areas for City of Tallahassee public-supply wells, Tallahassee, Florida

Ground water from the Upper Floridan aquifer is the sole source of water supply for Tallahassee, Florida, and the surrounding area. The City of Tallahassee (the City) currently operates 28 water-supply wells; 26 wells are distributed throughout the City and 2 are located in Woodville, Florida. Most of these wells yield an ample supply of potable water; however, water from several wells has low levels of tetrachloroethylene (PCE). The City removes the PCE from the water by passing it through granular-activated carbon units before distribution. To ensure that water-supply wells presently free of contamination remain clean, it is necessary to understand the ground-water flow system in sufficient detail to protect the contributing areas. Ground-water samples collected from four public-supply wells were analyzed for tritium ( 3 H), chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF 6 ). Using data for the CFC compounds, apparent ground-water ages ranged from 7 to 31 years. For SF 6 , the apparent ages tended to be about 5 to 10 years younger than those from CFCs. Apparent ages based on the tritium/tritiogenic helium-3 ( 3 H/ 3 He trit ) method ranged from 26 to 33 years. The three dating methods indicate that the apparent age of ground water generally decreases from northern to southern Leon County. This southward trend of decreasing ages is consistent with increasing amounts of recharge that occur as ground water moves from north to south. The ground-water age data derived by geochemical and tracer analyses were used in combination with the flow model and particle tracking to determine an effective porosity for the Hawthorn clays and Upper Floridan aquifer. The effective porosities for the Upper Floridan aquifer that resulted in best model matches were averaged to produce an effective porosity of 7 percent, and the effective porosities for the Hawthorn clays that resulted in a match were averaged to produce an effective porosity of 22 percent. Probabilistic contributing areas were determined for 26 City wells using MODFLOW and MODPATH. For each probabilistic contributing area delineated, the model was run 100 times and the results were analyzed statistically. For each of the 100 runs, a different hydraulic conductivity for each of the zones was assigned to the Upper Floridan aquifer. The hydraulic conductivities were generated randomly assuming a lognormal probability distribution; the mean of the distribution was equal to the hydraulic conductivity from the calibrated model. The 5-year time-dependent capture zones (TDCZs), assuming effective porosities of 0.1, 1, and 7 percent for four representative wells, were delineated. The higher probabilities of capture (greater than 40, 60, and 80 percent) were similar for all effective porosities, and the TDCZ delineated using a 7-percent porosity was slightly smaller; the lower probabilities of capture (greater than 10 and 20 percent) showed a large range of variability.

Florida↗

USGS2007

Chemical Characteristics, Water Sources and Pathways, and Age Distribution of Ground Water in the Contributing Recharge Area of a Public-Supply Well near Tampa, Florida, 2002-05

In 2001, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey began a series of studies on the transport of anthropogenic and natural contaminants (TANC) to public-supply wells. The main goal of the TANC program was to better understand the source, transport, and receptor factors that control contaminant movement to public-supply wells in representative aquifers of the United States. Studies were first conducted at regional scales at four of the eight TANC study areas during 2002-03 and at small (local) scales during 2003-05 in California, Nebraska, Connecticut, and Florida. In the Temple Terrace study area near Tampa, Florida, multiple chemical indicators and geochemical and ground-water flow modeling techniques were used to assess the vulnerability of a public-supply well in the karstic Upper Floridan aquifer to contamination from anthropogenic and naturally occurring contaminants. During 2003-05, water samples were collected from the public-supply well and 13 surrounding monitoring wells that all tap the Upper Floridan aquifer, and from 15 monitoring wells in the overlying surficial aquifer system and the intermediate confining unit that are located within the modeled ground-water contributing recharge area of the public-supply well. Six volatile organic compounds and four pesticides were detected in trace concentrations (well below drinking-water standards) in water from the public-supply well, which had an open interval from 36 to 53 meters below land surface. These contaminants were detected more frequently in water samples from monitoring wells in the overlying clastic surficial aquifer system than in water from monitoring wells in the Upper Floridan aquifer in the study area. Likewise, nitrate-N concentrations in the public-supply well (0.72-1.4 milligrams per liter) were more similar to median concentrations in the oxic surficial aquifer system (2.1 milligrams per liter) than to median nitrate-N concentrations in the anoxic Upper Floridan aquifer (0.06 milligram per liter) under sulfate-reducing conditions. High concentrations of radon-222 and uranium in the public-supply well compared to those in monitoring wells in the Upper Floridan aquifer appear to originate from water moving downward through sands and discontinuous clay lenses that overlie the aquifer. Water samples also were collected from three overlapping depth intervals (38-53, 43-53, and 49-53 meters below land surface) in the public-supply well. The 49- to 53-meter interval was identified as a high-flow zone during geophysical logging of the wellbore. Water samples were collected from these depth intervals at a low pumping rate by placing a low-capacity submersible pump (less than 0.02 cubic meter per minute) at the top of each interval. To represent higher pumping conditions, a large-capacity portable submersible pump (1.6 cubic meters per minute) was placed near the top of the open interval; water-chemistry samples were collected using the low-capacity submersible pump. The 49- to 53-meter depth interval had distinctly different chemistry than the other two sampled intervals. Higher concentrations of nitrate-N, atrazine, radon, trichloromethane (chloroform), and arsenic (and high arsenic (V)/arsenic (III) ratios); lower concentrations of dissolved solids, strontium, iron, manganese, and lower nitrogen and sulfur isotope ratios were found in this highly transmissive zone in the limestone than in water from the two other depth intervals. Movement of water likely occurs from the overlying sands and clays of the oxic surficial aquifer system and intermediate confining unit (that contains high radon-222 and nitrate-N concentrations) into the anoxic Upper Floridan aquifer (that contains low radon-222 and nitrate-N concentrations). Differences in arsenic concentrations in water from the various depth intervals in the public-supply well (3.2-19.0 micrograms per liter) were related to pumping conditions. The high arsenic

Scientific Investigations Report↗