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Neil M. Dubrovsky

Publications and source records attributed to Neil M. Dubrovsky.

18 recordsLinked to original sources
USGS2022

Quality of groundwater used for public supply in the continental United States: A comprehensive assessment

The presence of contaminants in a source water can constrain its suitability for drinking. The quality of groundwater used for public supply was assessed in 25 principal aquifers (PAs) that account for 84% of groundwater pumped for public supply in the U.S. (89.6 million people on a proportional basis). Each PA was sampled across its lateral extent using an equal-area grid, typically with 60 wells per PA. Samples were analyzed for 502 constituents, of which 374 had either a regulatory or nonregulatory human health benchmark (HHB). Nationally, elevated concentrations (relative to HHBs) of geogenic constituents have a larger effect than anthropogenic constituents, as indicated by three metrics: detection frequency, 35% versus 8.1%; prevalence (based on area), 41% versus 6.4%; and population potentially affected, 31.2 million versus 7.1 million. Prevalence of any constituent at elevated concentrations was high─40 to 75%─in PAs comprising unconsolidated sediment (eight PAs) and sandstone or interbedded sandstones and carbonates (four PAs) in the West and Central Interior. Prevalence was lower─15 to 35%─in PAs comprising sediment and sedimentary rocks along the Gulf and Atlantic Coasts (four PAs), carbonates distributed across the continental U.S. (seven PAs), and hard rock (two PAs).

continental United States↗
USGS2021

Lithium in groundwater used for drinking-water supply in the United States

Lithium concentrations in untreated groundwater from 1464 public-supply wells and 1676 domestic-supply wells distributed across 33 principal aquifers in the United States were evaluated for spatial variations and possible explanatory factors. Concentrations nationwide ranged from <1 to 396 μg/L (median of 8.1) for public supply wells and <1 to 1700 μg/L (median of 6 μg/L) for domestic supply wells. For context, lithium concentrations were compared to a Health Based Screening Level (HBSL, 10 μg/L) and a drinking-water only threshold (60 μg/L). These thresholds were exceeded in 45% and 9% of samples from public-supply wells and in 37% and 6% from domestic-supply wells, respectively. However, exceedances and median concentrations ranged broadly across geographic regions and principal aquifers. Concentrations were highest in arid regions and older groundwater, particularly in unconsolidated clastic aquifers and sandstones, and lowest in carbonate-rock aquifers, consistent with differences in lithium abundance among major lithologies and rock weathering extent. The median concentration for public-supply wells in the unconsolidated clastic High Plains aquifer (central United States) was 24.6 μg/L; 24% of the wells exceeded the drinking-water only threshold and 86% exceeded the HBSL. Other unconsolidated clastic aquifers in the arid West had exceedance rates comparable to the High Plains aquifer, whereas no public supply wells in the Biscayne aquifer (southern Florida) exceeded either threshold, and the highest concentration in that aquifer was 2.6 μg/L. Multiple lines of evidence indicate natural sources for the lithium concentrations; however, anthropogenic sources may be important in the future because of the rapid increase of lithium battery use and subsequent disposal. Geochemical models demonstrate that extensive evaporation, mineral dissolution, cation exchange, and mixing with geothermal waters or brines may account for the observed lithium and associated constituent concentrations, with the latter two processes as major contributing factors.

Science of the Total Environment↗
USGS2017

Large decadal-scale changes in uranium and bicarbonate in groundwater of the irrigated western U.S

Samples collected about one decade apart from 1105 wells from across the U.S. were compiled to assess whether uranium concentrations in the arid climate are linked to changing bicarbonate concentrations in the irrigated western U.S. Uranium concentrations in groundwater were high in the arid climate in the western U.S, where uranium sources are abundant. Sixty-four wells (6%) were above the U.S. EPA MCL of 30 μg/L; all but one are in the arid west. Concentrations were low to non-detectable in the humid climate. Large uranium and bicarbonate increases (differences are greater than the uncertainty in concentrations) occur in 109 wells between decade 1 and decade 2. Similarly, large uranium and bicarbonate decreases occur in 76 wells between the two decades. Significantly more wells are concordant (uranium and bicarbonate are both going the same direction) than discordant (uranium and bicarbonate are going opposite directions) (p < 0.001; Chi-square test). The largest percent difference in uranium concentrations occur in wells where uranium is increasing and bicarbonate is also increasing. These large differences occur mostly in the arid climate. Results are consistent with the hypothesis that changing uranium concentrations are linked to changes in bicarbonate in irrigated areas of the western U.S.

Science of the Total Environment↗
USGS2016

Cyanobacterial harmful algal blooms and U.S. Geological Survey science capabilities

Cyanobacterial harmful algal blooms (CyanoHABs) are increasingly a global concern because CyanoHABs pose a threat to human and aquatic ecosystem health and cause economic damages. Despite advances in scientific understanding of cyanobacteria and associated compounds, many unanswered questions remain about occurrence, environmental triggers for toxicity, and the ability to predict the timing, duration, and toxicity of CyanoHABs. U.S. Geological Survey (USGS) scientists are leading a diverse range of studies to address CyanoHAB issues in water bodies throughout the United States, using a combination of traditional methods and emerging technologies, and in collaboration with numerous partners. By providing practical applications of cutting edge CyanoHAB research, USGS studies have advanced scientific understanding, enabling the development of approaches to help protect ecological and human health.

Open-File Report↗
USGS2012

Assessment of regional change in nitrate concentrations in groundwater in the Central Valley, California, USA, 1950s-2000s

A regional assessment of multi-decadal changes in nitrate concentrations was done using historical data and a spatially stratified non-biased approach. Data were stratified into physiographic subregions on the basis of geomorphology and soils data to represent zones of historical recharge and discharge patterns in the basin. Data were also stratified by depth to represent a shallow zone generally representing domestic drinking-water supplies and a deep zone generally representing public drinking-water supplies. These stratifications were designed to characterize the regional extent of groundwater with common redox and age characteristics, two factors expected to influence changes in nitrate concentrations over time. Overall, increasing trends in nitrate concentrations and the proportion of nitrate concentrations above 5 mg/L were observed in the east fans subregion of the Central Valley. Whereas the west fans subregion has elevated nitrate concentrations, temporal trends were not detected, likely due to the heterogeneous nature of the water quality in this area and geologic sources of nitrate, combined with sparse and uneven data coverage. Generally low nitrate concentrations in the basin subregion are consistent with reduced geochemical conditions resulting from low permeability soils and higher organic content, reflecting the distal portions of alluvial fans and historical groundwater discharge areas. Very small increases in the shallow aquifer in the basin subregion may reflect downgradient movement of high nitrate groundwater from adjacent areas or overlying intensive agricultural inputs. Because of the general lack of regionally extensive long-term monitoring networks, the results from this study highlight the importance of placing studies of trends in water quality into regional context. Earlier work concluded that nitrate concentrations were steadily increasing over time in the eastern San Joaquin Valley, but clearly those trends do not apply to other physiographic subregions within the Central Valley, even where land use and climate are similar.

California↗
USGS2010

Deducing the distribution of terminal electron-accepting processes in hydrologically diverse groundwater systems

The distribution of microbially mediated terminal electron-accepting processes (TEAPs( was investigated in four hydrologically diverse groundwater systems by considering patterns of electron acceptor (nitrate, sulfate) consumption, intermediate product (hydrogen (H 2 )) concentrations, and final product (ferrous iron, sulfide, and methane) production. In each hydrologic system a determination of predominant TEAPs could be arrived at, but the level of confidence appropriate for each determination differed. In a portion of the lacustrine aquifer of the San Joaquin Valley, for example, all three indicators (sulfate concentrations decreasing, H 2 concentrations in the 1–2 nmol range, and sulfide concentrations increasing along flow paths identified sulfate reduction as the predominant TEAP, leading to a high degree of confidence in the determination. In portions of the Floridan aquifer and a petroleum hydrocarbon-contaminated aquifer, sulfate reduction and methanogenesis are indicated by production of sulfide and methane, and hydrogen oncentrations in the 1–4 nmol and 5–14 nmol range, respectively. However, because electron acceptor consumption could not be documented in these systems, less confidence is warranted in the TEAP determination. In the Black Creek aquifer, no pattern of sulfate consumption and sulfide production were observed, but H2 concentrations indicated sulfate reduction as the predominant TEAP. In this case, where just a single line of evidence is available, the least confidence in the TEAP diagnosis is justified. Because this methodology is based on measurable water chemistry parameters and upon the physiology of microbial electron transfer processes, it provides a better description of predominant redox processes in groundwater systems than more traditional Eh-based methods.

Water Resources Research↗
USGS2010

Nitrate in groundwater of the United States, 1991-2003

An assessment of nitrate concentrations in groundwater in the United States indicates that concentrations are highest in shallow, oxic groundwater beneath areas with high N inputs. During 1991-2003, 5101 wells were sampled in 51 study areas throughout the U.S. as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program. The well networks reflect the existing used resource represented by domestic wells in major aquifers (major aquifer studies), and recently recharged groundwater beneath dominant land-surface activities (land-use studies). Nitrate concentrations were highest in shallow groundwater beneath agricultural land use in areas with well-drained soils and oxic geochemical conditions. Nitrate concentrations were lowest in deep groundwater where groundwater is reduced, or where groundwater is older and hence concentrations reflect historically low N application rates. Classification and regression tree analysis was used to identify the relative importance of N inputs, biogeochemical processes, and physical aquifer properties in explaining nitrate concentrations in groundwater. Factors ranked by reduction in sum of squares indicate that dissolved iron concentrations explained most of the variation in groundwater nitrate concentration, followed by manganese, calcium, farm N fertilizer inputs, percent well-drained soils, and dissolved oxygen. Overall, nitrate concentrations in groundwater are most significantly affected by redox conditions, followed by nonpoint-source N inputs. Other water-quality indicators and physical variables had a secondary influence on nitrate concentrations.

Environmental Science & Technology↗
USGS2010

The quality of our nation's waters: Nutrients in the nation's streams and groundwater, 1992-2004

National Findings and Their Implications Although the use of artificial fertilizer has supported increasing food production to meet the needs of a growing population, increases in nutrient loadings from agricultural and, to a lesser extent, urban sources have resulted in nutrient concentrations in many streams and parts of aquifers that exceed standards for protection of human health and (or) aquatic life, often by large margins. Do NAWQA findings substantiate national concerns for aquatic and human health? National Water-Quality Assessment (NAWQA) findings indicate that nutrient concentrations in streams and groundwater in basins with significant agricultural or urban development are substantially greater than naturally occurring or “background” levels. For example, median concentrations of total nitrogen and phosphorus in agricultural streams are about 6 times greater than background levels. Findings also indicate that concentrations in streams routinely were 2 to 10 times greater than regional nutrient criteria recommended by the U.S. Environmental Protection Agency (USEPA) to protect aquatic life. Such large differences in magnitude suggest that significant reductions in sources of nutrients, as well as greater use of land management strategies to reduce the transport of nutrients to streams, are needed to meet recommended criteria for streams draining areas with significant agricultural and urban development. Nitrate concentrations above the Federal drinking-water standard—or Maximum Contaminant Level (MCL)—of 10 milligrams per liter (mg/L, as nitrogen) are relatively uncommon in samples from streams used for drinking water or from relatively deep aquifers; the MCL is exceeded, however, in more than 20 percent of shallow (less than 100 feet below the water table) domestic wells in agricultural areas. This finding raises concerns for human health in rural agricultural areas where shallow groundwater is used for domestic supply and may warn of future contamination of deeper groundwater pumped from public‑supply wells. Are levels of nutrients in water increasing or decreasing? A decadal assessment of trends in concentrations of nitrogen and phosphorus from about 1993 to 2003 shows minimal changes in those concentrations in the majority of studied streams across the Nation, and more upward than downward trends in concentrations at sites with changes. These findings underscore the need for reductions in nutrient inputs or management strategies that would reduce transport of nutrients to streams. Upward trends were evident among all land uses, including those only minimally affected by agricultural and (or) urban development, which suggests that additional protection of some of our Nation’s most pristine streams warrants consideration. The median of nitrate concentrations in groundwater from 495 wells also increased significantly from 3.2 to 3.4 mg/L (6 percent) during about the same period, and the proportion of wells with concentrations of nitrate greater than the MCL increased from 16 to 21 percent. Nitrate concentrations in water in deep aquifers are likely to increase during the next decade as shallow groundwater with elevated concentrations moves downward. The potential for future contamination of the deep aquifers requires attention because these aquifers commonly are used for public water supply, and because restoration of groundwater is costly and difficult. Long-term and consistent monitoring of nutrients, improved accounting of nutrient sources, and improved tracking and modeling of climatic and landscape changes will be essential for distinguishing trends in nutrient concentrations, understanding the causes of those trends, and accurately tracking the effectiveness of strategies implemented to manage nutrients.

Circular↗
USGS2010

Nutrients in the nation's streams and groundwater: National findings and implications

A comprehensive national analysis of the distribution and trends of nutrient concentrations in streams and groundwater from 1992 through 2004 is provided by the National Water-Quality Assessment (NAWQA) Program of the United States Geological Survey (USGS). Findings describe the distribution and causes of varying nutrient concentrations in streams and groundwater throughout the Nation and examine the primary sources that contribute to elevated concentrations. Results show that excessive nutrient enrichment is a widespread cause of ecological degradation in streams and that nitrate contamination of groundwater used for drinking water, particularly shallow domestic wells in agricultural areas, is a continuing human-health concern. Finally, despite major Federal, State and local nonpoint-source nutrient control efforts for streams and watersheds across the Nation, USGS trend analyses for 1993?2003 suggest limited national progress to reduce the impacts of nonpoint sources of nutrients during this period. Instead, concentrations have remained the same or increased in many streams and aquifers across the Nation, and continue to pose risks to aquatic life and human health. This Fact Sheet highlights selected national findings and their implications, and serves as a companion product to the complete analysis reported in the USGS Circular titled ?The Quality of Our Nation?s Waters?Nutrients in the Nation?s Streams and Groundwater, 1992?2004.?

Fact Sheet↗
USGS2010

Nitrate Loads and Concentrations in Surface-Water Base Flow and Shallow Groundwater for Selected Basins in the United States, Water Years 1990-2006

Hydrograph separation was used to determine the base-flow component of streamflow for 148 sites sampled as part of the National Water-Quality Assessment program. Sites in the Southwest and the Northwest tend to have base-flow index values greater than 0.5. Sites in the Midwest and the eastern portion of the Southern Plains generally have values less than 0.5. Base-flow index values for sites in the Southeast and Northeast are mixed with values less than and greater than 0.5. Hypothesized flow paths based on relative scaling of soil and bedrock permeability explain some of the differences found in base-flow index. Sites in areas with impermeable soils and bedrock (areas where overland flow may be the primary hydrologic flow path) tend to have lower base-flow index values than sites in areas with either permeable bedrock or permeable soils (areas where deep groundwater flow paths or shallow groundwater flow paths may occur). The percentage of nitrate load contributed by base flow was determined using total flow and base flow nitrate load models. These regression-based models were calibrated using available nitrate samples and total streamflow or base-flow nitrate samples and the base-flow component of total streamflow. Many streams in the country have a large proportion of nitrate load contributed by base flow: 40 percent of sites have more than 50 percent of the total nitrate load contributed by base flow. Sites in the Midwest and eastern portion of the Southern Plains generally have less than 50 percent of the total nitrate load contributed by base flow. Sites in the Northern Plains and Northwest have nitrate load ratios that generally are greater than 50 percent. Nitrate load ratios for sites in the Southeast and Northeast are mixed with values less than and greater than 50 percent. Significantly lower contributions of nitrate from base flow were found at sites in areas with impermeable soils and impermeable bedrock. These areas could be most responsive to nutrient management practices designed to reduce nutrient transport to streams by runoff. Conversely, sites with potential for shallow or deep groundwater contribution (some combination of permeable soils or permeable bedrock) had significantly greater contributions of nitrate from base flow. Effective nutrient management strategies would consider groundwater nitrate contributions in these areas. Mean annual base-flow nitrate concentrations were compared to shallow-groundwater nitrate concentrations for 27 sites. Concentrations in groundwater tended to be greater than base-flow concentrations for this group of sites. Sites where groundwater concentrations were much greater than base-flow concentrations were found in areas of high infiltration and oxic groundwater conditions. The lack of correspondingly high concentrations in the base flow of the paired surface-water sites may have multiple causes. In some settings, there has not been sufficient time for enough high-nitrate shallow groundwater to migrate to the nearby stream. In these cases, the stream nitrate concentrations lag behind those in the shallow groundwater, and concentrations may increase in the future as more high-nitrate groundwater reaches the stream. Alternatively, some of these sites may have processes that rapidly remove nitrate as water moves from the aquifer into the stream channel. Partitioning streamflow and nitrate load between the quick-flow and base-flow portions of the hydrograph coupled with relative scales of soil permeability can infer the importance of surface water compared to groundwater nitrate sources. Study of the relation of nitrate concentrations to base-flow index and the comparison of groundwater nitrate concentrations to stream nitrate concentrations during times when base-flow index is high can provide evidence of potential nitrate transport mechanisms. Accounting for the surface-water and groundwater contributions of nitrate is crucial to effective management and remediat

Scientific Investigations Report↗
USGS2007

Trace-Element Concentrations in Tissues of Aquatic Organisms from Rivers and Streams of the United States, 1992-1999

The U.S. Geological Survey National Water-Quality Assessment Program collected tissue samples from a variety of aquatic organisms during 1992-1999 within 47 study units across the United States. These tissue samples were collected to determine the occurrence and distribution of 20 major and minor trace elements in aquatic organisms. This report presents the tissue trace-element concentration data, sample summaries, and concentration statistics for 1,457 tissue samples representing 76 species or groups of fish, aquatic invertebrates, and plants were collected at 824 sampling sites.

Data Series↗
USGS2001

Low-Level Volatile Organic Compounds in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

Data were collected to evaluate the use of low-level volatile organic compounds (VOC) to assess the vulnerability of public supply wells in the Los Angeles physiographic basin. Samples of untreated ground water from 178 active public supply wells in the Los Angeles physiographic basin show that VOCs were detected in 61 percent of the ground-water samples; most of these detections were low, with only 29 percent above 1 mg/L (microgram per liter). Thirty-nine of the 86 VOCs analyzed were detected in at least one sample, and 11 VOCs were detected in 7 percent or more of the samples. The six most frequently detected VOCs were trichloromethane (chloroform) (46 percent); trichloroethene (TCE) (28 percent); tetrachloro-ethene (PCE) (19 percent); methyl tert-butyl ether (MTBE) (14 percent); 1,1-dichloroethane (11 percent); and 1,1,1-trichloroethane (TCA) (11 percent). These VOCs were also the most frequently detected VOCs in ground water representative of a wide range of hydrologically conditions in urban areas nationwide. Only two VOCs (TCE and PCE) exceeded state and federal primary maximum contaminant levels (MCL) for drinking water in a total of seven samples. Because samples were collected prior to water treatment, sample concentrations do not represent the concentrations entering the drinking-water system.Ground water containing VOCs may be considered to be a tracer of postindustrial-aged water-water that was recharged after the onset of intense urban development. The overall distribution of VOC detections is related to the hydrological and the engineered recharge facilities in the Coastal Los Angeles Basin and the Coastal Santa Ana Basin that comprise the Los Angeles physiographic basin. Most of the ground-water recharge occurs at engineered recharge facilities in the generally coarse-grained northeastern parts of the study area (forebay areas). Ground-water recharge from the land surface is minimal in the southwestern part of the basins, distal from the recharge facilities, where clay layers impede the vertical migration of ground water (pressure areas).VOCs are not uniformly distributed over the study area. Most of the wells with multiple VOC detections, which also have the highest concentrations, are in the forebay areas and are clustered proximal to the recharge facilities. In addition, the number of VOC detections and VOC concentrations decrease beyond about 10-15 kilometers from the recharge facilities. The distribution of individual VOCs is also related to their history of use. MTBE traces ground water recharged during about the last decade and is detected almost exclusively in the forebay areas. Chloroform, which has been used since the 1920s, is more widely distributed and is detected at the greatest distances from the recharge facilities.Downward migration of VOCs from the land surface may be a viable process for VOCs to reach aquifers in parts of the forebay areas, but there is little indication that the same process is active in the pressure area. The lack of contrast in the number of VOC detections between wells of different depths over most of the study area suggests that the downward migration from the land surface is not a dominant pathway for VOCs to travel to the capture zones of public supply wells. Isolated occurrences of multiple VOC detections and high concentrations of VOCs in individual wells may indicate rapid vertical transport from a localized source. Stable isotope data indicate that ground water containing VOCs is a mixture of local precipitation and runoff with water that is isotopically lighter (more negative) than the local sources. The isotopically lighter water could either be Colorado River water or State Water Project water, both of which are imported to the basin and used as a source of recharge to the ground-water flow system. The stable isotope data support the interpretation that VOCs in ground water are associated with the engineered recharge facilities.Two of the most frequently detecte

Water-Resources Investigations Report↗
USGS1998

Water quality in the San Joaquin-Tulare basins, California, 1992-95

This report is intended to summarize major findings that emerged between 1992 and 1995 from the water-quality assessment of the San Joaquin-Tulare Basins Study Unit and to relate these findings to water-quality issues of regional and national concern. The information is primarily intended for those who are involved in water-resource management. Yet, the information contained here may also interest those who simply wish to know more about the quality of water in the rivers and aquifers in the area where they live.

California↗
USGS1998

Environmental setting of the San Joaquin-Tulare basins, California

The National Water-Quality Assessment Program for the San Joaquin-Tulare Basins began in 1991 to study the effects of natural and anthropogenic influences on the quality of ground water, surface water, biology, and ecology. The San Joaquin-Tulare Basins study unit, which covers approximately 31,200 square miles in central California, is made up of the San Joaquin Valley, the eastern slope of the Coast Ranges to the west, and the western slope of the Sierra Nevada to the east. The sediments of the San Joaquin Valley can be divided into alluvial fans and basin deposits. The San Joaquin River receives water from tributaries draining the Sierra Nevada and Coast Ranges, and except for streams discharging directly to the Sacramento-San Joaquin Delta, is the only surface- water outlet from the study unit. The surface-water hydrology of the San Joaquin-Tulare Basins study unit has been significantly modified by development of water resources. Almost every major river entering the valley from the Sierra Nevada has one or more reservoirs. Almost every tributary and drainage into the San Joaquin River has been altered by a network of canals, drains, and wasteways. The Sierra Nevada is predominantly forested, and the Coast Ranges and the foothills of the Sierra Nevada are predominately rangeland. The San Joaquin Valley is dominated by agriculture, which utilized approximately 14.7 million acre-feet of water and 597 million pounds active ingredient of nitrogen and phosphorus fertilizers in 1990, and 88 million pounds active ingredient of pesticides in 1991. In addition, the livestock industry contributed 318 million pounds active ingredient of nitrogen and phosphorus from manure in 1987. This report provides the background information to assess the influence of these and other factors on water quality and to provide the foundation for the design and interpretation of all spatial data. These characterizations provide a basis for comparing the influences of human activities among basins and specific land use settings, as well as within and among study units at the national level.

California↗
USGS1998

Occurrence of nitrate and pesticides in ground water beneath three agricultural land-use settings in the eastern San Joaquin Valley, California, 1993-1995

The processes that affect nitrate and pesticide occurrence may be better understood by relating ground-water quality to natural and human factors in the context of distinct, regionally extensive, land- use settings. This study assesses nitrate and pesticide occurrence in ground water beneath three agricultural land-use settings in the eastern San Joaquin Valley, California. Water samples were collected from 60 domestic wells in vineyard, almond, and a crop grouping of corn, alfalfa, and vegetable land-use settings. Each well was sampled once during 1993?1995. This study is one element of the U.S. Geological Survey?s National Water-Quality Assessment Program, which is designed to assess the status of, and trends in, the quality of the nation?s ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. The concentrations and occurrence of nitrate and pesticides in ground-water samples from domestic wells in the eastern alluvial fan physiographic region were related to differences in chemical applica- tions and to the physical and biogeochemical processes that charac- terize each of the three land-use settings. Ground water beneath the vineyard and almond land-use settings on the coarse-grained, upper and middle parts of the alluvial fans is more vulnerable to nonpoint- source agricultural contamination than is the ground water beneath the corn, alfalfa, and vegetable land-use setting on the lower part of the fans, near the basin physiographic region. Nitrate concentrations ranged from less than 0.05 to 55 milligrams per liter, as nitrogen. Nitrate concentrations were significantly higher in the almond land-use setting than in the vineyard land-use setting, whereas concentrations in the corn, alfalfa, and vegetable land-use setting were intermediate. Nitrate concentrations exceeded the maximum contaminant level in eight samples from the almond land- use setting (40 percent), in seven samples from the corn, alfalfa, and vegetable land-use setting (35 percent), and in three samples from the vineyard land-use setting (15 percent). The physical and chemical characteristics of the vineyard and the almond land-use settings are similar, characterized by coarse-grained sediments and high dissolved- oxygen concentrations, reflecting processes that promote rapid infiltration of water and solutes. The high nitrate concentrations in the almond land-use setting reflect the high amount of nitrogen appli- cations in this setting, whereas the low nitrate concentrations in the vineyard land-use setting reflect relatively low nitrogen applications. In the corn, alfalfa, and vegetable land-use setting, the relatively fine-grained sediments, and low dissolved-oxygen concentrations, reflect processes that result in slow infiltration rates and longer ground-water residence times. The intermediate nitrate concentrations in the corn, alfalfa, and vegetable land-use setting are a result of these physical and chemical characteristics, combined with generally high (but variable) nitrogen applications. Twenty-three different pesticides were detected in 41 of 60 ground- water samples (68 percent). Eighty percent of the ground-water samples from the vineyard land-use setting had at least one pesticide detection, followed by 70 percent in the almond land-use setting, and 55 percent in the corn, alfalfa, and vegetable land-use setting. All concentra- tions were less than state or federal maximum contaminant levels only 5 of the detected pesticides have established maximum contaminant levels) with the exception of 1,2-dibromo-3-chloropropane, which exceeded the maximum contaminant level of 0.2 micrograms per liter in 10 ground-water samples from vineyard land-use wells and in 5 ground- water samples from almond land-use wells. Simazine was detected most often, occurring in 50 percent of the ground-water samples from the vineyard land-use wells and in 30 percent

Water-Resources Investigations Report↗
USGS1998

Occurrence and distribution of dissolved pesticides in the San Joaquin River basin, California

The effects of pesticide application, hydrology, and chemical and physical properties on the occurrence of pesticides in surface water in the San Joaquin River Basin, California, were examined. The study of pesticide occurrence in the highly agricultural San Joaquin?Tulare Basins is part of the National Water-Quality Assessment Program of the U.S. Geological Survey. One hundred forty-three water samples were collected throughout 1993 from sites on the San Joaquin River and three of its tributaries: Orestimba Creek, Salt Slough, and the Merced River. Of the 83 pesticides selected for analysis in this study, 49 different compounds were detected in samples from the four sites and ranged in concentration from less than the detection limit to 20 micrograms per liter. All but one sample contained at least one pesticide, and more than 50 percent of the samples contained seven or more pesticides. Six compounds were detected in more than 50 percent of the samples: four herbicides (dacthal, EPTC, metolachlor, and simazine) and two insecticides (chlorpyrifos and diazinon). None of the measured concentrations exceeded U.S. Environmental Protection Agency drinking water criteria, and many of the measured concentrations were very low. The concentrations of seven pesticides exceeded criteria for the protection of freshwater aquatic life: azinphos-methyl, carbaryl, chlorpyrifos, diazinon, diuron, malathion, and trifluralin. Overall, some criteria for protection of aquatic life were exceeded in a total of 97 samples. Factors affecting the spatial patterns of occurrence of the pesticides in the different subbasins included the pattern of application and hydrology. Seventy percent of pesticides with known application were detected. Overall, 40 different pesticides were detected in Orestimba Creek, 33 in Salt Slough, and 26 in the Merced River. Samples from the Merced River had a relatively low number of detections, despite the high number (35) of pesticides applied, owing to the generally low percentage of irrigation return flow and contribution of pesticide-free streamflow from reservoir releases. Irrigation return flows in the Orestimba Creek and Salt Slough subbasins generally contained more pesticides at higher concentrations. In addition, the distribution of seven pesticides (alachlor, cyanazine, dacthal, fonofos, molinate, napropamide, and trifluralin) in the subbasins showed a direct spatial correspondence between occurrence and application rates. Temporal patterns of occurrence also were affected by patterns of application and hydrology. Most pesticides showed a clear correspondence between the times of their application and their occurrence. Fourteen pesticides had maximum application and concentrations during the summer irrigation season. However, several pesticides exhibited maximum concentrations during winter storms, although maximum application occurred at some other time of year?the result of differences in precipitation and streamflow between seasons. In some subbasins, precipitation runoff was more effective than irrigation return flows at transporting pesticides from the site of application to the stream. Also, during autumn, when there was neither precipitation nor irrigation, the transport of pesticides to streams was limited. The effect of chemical and physical properties on the occurrence of pesticides was examined for the San Joaquin River Basin as a whole. The runoff potential of each pesticide, calculated from the solubility, water-soil organic carbon partition coefficient Koc, and hydrolysis half-life, is generally consistent with the frequency of detection of pesticides in surface water in relation to the amount applied. These three properties each were generally, and weakly, correlated with the relative load of the pesticides in surface water. Pesticide occurrence and concentrations at the mouth of the basin (the San Joaquin River near Vernalis) was compared with pesticide occurrence and concentration in the three

Water-Resources Investigations Report↗
USGS1995

Inputs of the Dormant-Spray Pesticide, Diazinon, to the San Joaquin River, California, February 1993

INTRODUCTION The objective of the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey is to describe the status and trends of the Nation's water quality with respect to natural features of the environment and human activities or land-use. Pesticides are a major water-quality issue in the San Joaquin Valley of California (fig. 1), and pesticide residues may be transported to rivers and streams in agricultural runoff following winter storms. Three sites in the western San Joaquin Valley were monitored during and after two February 1993 storms. The storms occurred after extensive spraying of organophosphate insecticides, mostly diazinon, on almond and other stone-fruit orchards.

Fact Sheet↗