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USGS2024

Dissolved arsenic concentrations in surface waters within the upper portions of the Klamath River Basin, Oregon and California

Arsenic toxicity is an environmental health problem. Levels of arsenic in surface waters at some locations in the Klamath River Basin in southern Oregon and northern California can exceed the U.S. Environmental Protection Agency (EPA) standard for drinking water. There are both anthropogenic and natural sources of arsenic. The Klamath River Basin consists primarily of volcanic deposits and contains an underground geothermal system with hot springs and warm water wells, all known natural sources of arsenic. Anthropogenic sources of arsenic are related to the agricultural use of herbicides, fungicides, and insecticides. Surface water arsenic levels can also be affected by fertilizer amendments, evaporative concentration, oxygen-level depletion, and various geochemical transformations that can increase arsenic mobilization. In this study by the U.S. Geological Survey and the Bureau of Reclamation, dissolved concentrations of arsenic, copper, and lead were measured in surface waters at 39 unique sites within the upper portions of the Klamath River Basin between 2018 and 2022. In every year, except 2022, sites were sampled four times between April and November. Surface-water arsenic concentrations varied up to four-orders of magnitude among sites. Median arsenic concentration was lowest at Cherry Creek (0.03 micrograms per liter [μg/L]) and highest at Wood Kimball Spring (36.7 μg/L), two sites located north of Upper Klamath Lake. The highest arsenic concentrations (17.4±4.9 μg/L, n =3) were found in drain sites (defined here as a waterbody returning used irrigation water) while the lowest arsenic concentrations were found in an artesian well (0.8 μg/L, n =1). The elevated arsenic concentrations of the drain sites suggest that arsenic might be concentrated or mobilized by agricultural activities, water re-use practices, and (or) by geochemical processes occurring around water stored in drains (that is, in the water column and across sediment water boundaries). A source of arsenic in drain water in the Klamath Strait Drain area includes water used for irrigation originating from Ady Canal. Other potential sources include groundwater, geothermal water, and local soils and sediments. Seasonal differences in surface-water arsenic concentrations were detected at 13 sites, 10 of which had higher arsenic concentrations in summer than in either spring or fall. The sites sampled around Upper Klamath Lake, the impounded rivers, one of the two canal sites, and 5 of the 14 river sites had higher surface-water arsenic concentrations in the summer than in either spring or fall. Surface-water arsenic concentrations from groundwater sources (that is, springs and in the artesian well) did not vary significantly among seasons (p-values greater than 0.1). Median surface-water concentrations of copper and lead ranged from 0.03 to 3.7 μg/L, and from 0.013 to 0.175 μg/L ( n =2–18), respectively. Dissolved concentrations of both metals were below acute toxicity endpoints reported by the EPA for freshwater animals. Surface-water arsenic concentrations varied independently from corresponding changes in surface-water lead or copper concentrations. However, arsenic concentrations measured in bed-sediment samples collected from a subset of sites located north of Upper Klamath Lake correlated strongly and significantly with the corresponding sedimentary lead concentrations ( p =0.015). Aqueous arsenic speciation measured in a subset of sites in 2019 and 2022 showed that all the arsenic existed as arsenic (V), the most oxidized arsenic species, and presumably, the least toxic. The highest proportions of arsenite (As(III)), the presumably most toxic arsenic species, relative to total arsenic concentrations were found at drain sites. Our assessment of dissolved arsenic concentrations in various surface-water bodies in the Upper Klamath River Basin reveals geographical areas of consistently low (below 2.1 μg/L), moderate (below 10 μg/L) and high (above 10 μg/L) surface-water arsenic concentrations. South of Upper Klamath Lake, surface-water arsenic concentrations were consistently higher than 20 μg/L at two drain sites located in an area of predominant agricultural land use with extensive water re-use practices. North of Upper Klamath Lake, surface-water arsenic concentrations greater than 20 μg/L were consistently measured at sites with limited nearby agricultural activities, suggesting a geogenic source. The consistently high arsenic levels from the Wood River at Jackson F. Kimball State Park, Fort Creek, and Crooked Creek, which are sites located at or near headwater spring sources, suggest a natural background source of arsenic. Water flowing downstream from this area could be a potential source of arsenic to Upper Klamath Lake and the Upper Klamath River.

California, Oregon↗

USGS2024

Evidence of nitrate attenuation in intertidal and subtidal groundwater in a subterranean estuary at a Cape Cod embayment, East Falmouth, Massachusetts, 2015–16

Nitrogen dynamics in intertidal and nearshore subtidal groundwater (subterranean estuary) adjacent to the Seacoast Shores peninsula, Falmouth, Massachusetts, were investigated during 2015–16 by the U.S. Geological Survey. The peninsula is a densely populated residential area with septic systems and cesspools that are substantial sources of nitrogen to groundwater. The study area is in the Eel River, an estuarine saltwater embayment connected to the ocean adjacent to the western shore of the peninsula, that was the subject of an earlier study by Colman and others (2018, https://doi.org/10.3133/sir20185095 ) on nitrogen transport and transformations in groundwater between onshore and offshore locations. The previous study documented the distribution of nitrate concentrations and nitrate attenuation reactions in fresh groundwater beneath the peninsula and the estuary. The current study extended those observations with more detailed sampling and analysis of shallow groundwater from wells near discharge sites beneath the estuary. The current field investigation included sampling of existing wells and installation and sampling of clusters of wells and temporary sampling points in the subterranean estuary, including (1) shallow transects 0.3 to 1.2 meters (m) deep extending from 1 to 13.5 m offshore and (2) deeper wells (from 1.83 to 4.88 m deep) extending from 4.3 to 14.3 m offshore. Measurements of hydraulic-head gradients 2–5 m below the sediment/water interface in the intertidal and nearshore subtidal zones indicated that groundwater flow generally was upwards (towards the estuary) under all tide conditions in October 2016. The magnitude of the gradient was greatest during low tide conditions, indicating that groundwater discharge likely decreased during high tides. Measurements of specific conductance in shallow groundwater in the subterranean estuary in three transects perpendicular to shore were consistent with the existence of saltwater flow cells (infiltration of overlying saline water, mixing with fresh groundwater, and discharge to the overlying saline water) in the intertidal and nearshore subtidal regions. The size of these flow cells was variable in space and time and dependent on the elevation of the tide (spring or neap). At this location in the Eel River subterranean estuary, and offshore to at least 13.5 m, offshore flow of fresh groundwater apparently prevented a deeper saltwater wedge from discharging to the surface. Nitrate concentrations in shallow groundwater (30 to 122 centimeters [cm] depth) were variable in space and time, ranging from not detectable to 600 micromoles per liter (μmol/L) (8.4 milligrams per liter as N), and were highest in June 2016 at depths from 61 to 122 cm below the sediment/water interface and from 4 to 9 m offshore. Nitrate generally was not detectable in saline shallow groundwater at 30-cm depth or at any depth from 30 to 122 cm from 10 to 13.5 m offshore. Dissolved oxygen concentrations were suboxic (less than 16 μmol/L) in 60 percent of the sampled subterranean groundwater beneath the intertidal and subtidal zones. In the remaining sites, the range of dissolved oxygen concentrations was from 18 to 272 μmol/L and the median concentration was 43 μmol/L. Evidence for microbial nitrate reduction (denitrification and possibly anammox) was provided by the distribution of the reaction product nitrogen gas (excess N 2 , or N 2MIC ), as determined from analysis of the dissolved nitrogen gas and argon gas (Ar) concentrations in groundwater samples. Excess nitrogen gas provided evidence for nitrate reduction in shallow groundwater below the subtidal and, to a lesser extent, intertidal zones adjacent to the Seacoast Shores peninsula. These zones, where evidence for nitrate reduction was detected, were in fresh and brackish groundwater near subtidal or intertidal saltwater cells where discharging fresh groundwater mixed with infiltrating saline water. Infiltrating seawater may have supplied organic carbon, one of several potential electron donors that are required for denitrification. Other potential electron donors, such as organic carbon, iron, manganese, hydrogen, methane, ammonium, elemental sulfur, or sulfide phases, may have been supplied by the estuarine sediments. Drainage from surface runoff near the shore also may have supplied organic carbon to fresh groundwater near the intertidal saltwater cell. The highest amounts of nitrate converted to excess nitrogen gas were estimated to be in the range of 230 to 430 μmol/L in nearly fresh groundwater near the subtidal saltwater cell at depths of 61 to 122 cm below the sediment/water interface and from 10 to 13.5 m offshore. Evidence of denitrification within 10 m of the shore was sparse (generally limited to less than 50 μmol/L of N 2 -N) despite the presence of high nitrate concentrations. The spatial distribution of estimated nitrate reduction in the intertidal and nearshore subtidal fresh and brackish groundwater may be related to local variability in the distribution of reactive electron donors in those zones. Variations in the amount of nitrate reduction to nitrogen gas were not clearly related to potential aqueous electron donors such as dissolved organic carbon, nor to potential reaction products such as alkalinity, but may have been controlled by combinations of aqueous and solid-phase reactants. The distribution of relatively shallow fresh groundwater containing nitrate could indicate potential nitrate discharge areas in the lower intertidal zone and uncertain locations farther offshore; however, the data did not extend all the way to the sediment/water interface or to the offshore freshwater limit. This study confirmed substantial loss of nitrate from some of the fresh and brackish groundwater in shallow subestuarine sediments prior to discharge but did not quantify how much nitrate eventually discharged to the estuary.

Massachusetts↗

USGS2024

Simulation of groundwater flow and brine discharge to the Dolores River in the Paradox Valley, Montrose County, Colorado

Salinity, or total dissolved solids (TDS), of the Colorado River affects agricultural, municipal, and industrial water users and is an important concern in the Western United States. In the Paradox Valley of southwestern Colorado, natural discharge of sodium-chloride brine to the Dolores River from the underlying core of a salt-valley anticline accounts for about 6 percent of the salinity load to the Colorado River. Formation of the Paradox Valley began during the Miocene, and subsequent erosion exposed the Pennsylvania Paradox Formation in the core of the anticline where a cap rock, collapse features, breccia, and sodium-chloride saturated brine developed at the top of the exposed salt diapir. The discharge of brine to the Dolores River is affected by these dissolution features, along with seasonal hydrologic conditions and density-dependent flow between older dense brine and the younger fresh groundwater in the overlying alluvial aquifer. To reduce TDS concentrations in the Dolores River through the Paradox Valley, the Bureau of Reclamation has pumped brine from a series of shallow wells adjacent to the river since July 1996. The pumped brine is collected and piped to a deep disposal well where it is injected into the Mississippian Leadville Limestone at a depth of about 4,570-meters below land surface. The pumping and injection operation is collectively known as the Paradox Valley Unit (PVU), and by 2015, the PVU had substantially reduced TDS concentrations in the Dolores River by about 70 percent. Since 2019, injection-pressure limits and related seismic activity have constrained deep-well injection and thus brine pumping at the PVU. In cooperation with the Bureau of Reclamation, the U.S. Geological Survey developed a MODFLOW-6 three-dimensional, variable-density groundwater flow and TDS transport model of the Paradox Valley to evaluate the effects of PVU pumping operations on brine discharge to the Dolores River and to guide additional research. The finite-difference model grid consists of 76 rows and 48 columns oriented from northwest to southeast in alignment with valley topography and groundwater-flow directions in the near-surface freshwater alluvial aquifer. A 7-layer hydrogeologic framework was developed from existing datasets to represent the alluvial aquifer, cap rock, collapse breccia, and groundwater flow and TDS transport from the underlying Paradox Formation salt to the Dolores River. The model represents a 33-year transient calibration period from 1987 through 2020 that includes pre-PVU conditions from 1987 through June 1996 and post-PVU conditions from July 1996 through 2020. A 1,000-year simulation of groundwater flow and coupled TDS transport computed the initial conditions for the subsequent 33-year transient simulation. Observations of precipitation, streamflow, evaporation, agricultural land use, and PVU brine pumping rates were used to specify appropriate boundary conditions to the model representing time-varying recharge, tributary streamflow, groundwater underflow, evapotranspiration (ET), and PVU pumping. Values for average monthly streamflow and TDS concentration at the upstream streamgage, the Dolores River at Bedrock (USGS streamgage 09169500), were specified as model input where the Dolores River enters Paradox Valley. Observed pumping from the PVU, water levels and TDS concentrations in groundwater, and streamflow and estimated TDS concentrations at the downstream streamgage, the Dolores River near Bedrock (USGS streamgage 09171100), were calibration targets that constrained the manual calibration of model parameters representing aquifer hydraulic conductivity, storage, streambed conductance, recharge, and (ET). Two primary model-calibration targets were the match between observed and simulated TDS mass flux from PVU pumping wells and the match between estimated and simulated TDS mass flux to the Dolores River. The simulated TDS mass withdrawn by pumping wells is calculated by the model as the product of the assigned pumping rate and simulated groundwater TDS concentrations. Because actual pumping rates were assigned as simulated values, the total simulated PVU pumping for the 33-year calibration is within 0.5 percent of the observed values. However, simulated concentrations and thus mass flux of TDS withdrawn by the PVU pumping wells were consistently about 26 percent less than observed values for all the simulated time periods (33-year simulation, pre-PVU, and post-PVU). The representation of brine inflow was explored through additional modeling to evaluate the effect of the simulated brine source on groundwater TDS concentrations. Results indicated that a saturated-salt constant-flux brine source best replicated the magnitude and transient pattern observed for TDS mass flux from PVU pumping wells. The simulated TDS mass flux to the Dolores River is compared to estimates based on observed streamflow and specific conductance (SC) data for the downstream streamgage. The calibrated model provided a close fit of simulated to measured streamflow at the downstream streamgage, and the calibrated model fit to estimated TDS concentrations at the downstream streamgage was reasonable. The greatest differences between simulated and estimated values occurred during drought periods from June 2000 to March 2003, May 2012 to June 2013, and October 2013 to October 2014, when simulated TDS concentrations in the river were greater than estimated concentrations. In general, simulated TDS mass flux to the river for the pre-PVU period is in good agreement with estimated values (2-percent difference), but the model overestimated TDS mass flux to the river by about 41 percent during the post-PVU period. The model uncertainty with respect to TDS mass flux to the river indicates other processes or model parameters not well represented by the model are affecting the system, especially during drought. During model calibration, the most sensitive parameters were identified as vertical hydraulic conductivity of the alluvial aquifer, conductance of the Dolores River streambed, ET extinction depth and rate, and recharge rate. Five 5-year scenarios of conditions for 2021–25 were simulated to assist evaluation of alternative strategies to manage the discharge of brine into the Dolores River. The first scenario simulates no PVU pumping and serves as a base case for comparison to the other scenarios. Two scenarios simulate the effects of varying withdrawal timing at an annual rate about one-third less than during 2010 through 2018. During high-flow spring snowmelt runoff periods when brine discharge is naturally minimized, PVU pumping does not substantially affect salinity in the Dolores River, and comparison of these two scenarios indicates that scheduling brine withdrawals during times of low river stage is nearly as effective at reducing TDS mass flux to the river as pumping brine year-round. Cessation of pumping during periods of high river stage may be advantageous for system maintenance, brine injection, and seismic-risk reduction. The fourth scenario tested the effect of reducing irrigation-return flow on brine discharge and predicted a slight reduction of TDS mass flux to the Dolores River, but not as great a reduction as that of using the PVU to remove brine. The fifth scenario simulated 5 years of drought conditions without PVU pumping and indicates brine discharge during drought about 15 percent greater than during average hydrologic conditions. Results from scenario 5 are consistent with the calibrated model results and indicate that aquifer properties and ET processes and parameters may be affecting simulation results during drought. The Paradox Valley groundwater model provides a reasonable overall match to observed conditions in the Dolores River. The model is useful for evaluating relative differences between brine management scenarios to inform PVU operational decisions and to identify gaps in data and process understanding. Representation of the brine source, hydraulic-conductivity parameters, and recharge and ET processes were identified as potential areas for additional field and modeling research. Additional research in the Paradox Valley might include field-data collection that provides additional information on the hydrogeologic framework, groundwater levels, groundwater TDS concentrations, stream characteristics, and aquifer properties. Additional modeling efforts could benefit from applying advanced tools for model development, calibration, and visualization including parameter-estimation and sensitivity analysis. Statistical evaluation of known model uncertainties such as hydraulic conductivity, streambed conductance, representations of the brine source, recharge, and ET could improve the match between simulated and estimated TDS mass flux from PVU pumping wells and to the Dolores River further informing model predictions and system understanding for the Paradox Valley.

Colorado↗

USGS1973

Geohydrology of the Cross-Florida Barge Canal area, with special reference to the Ocala vicinity

The Cross-Florida Barge Canal route commences at Palatka on the St. Johns River, about 75 miles upstream from the Atlantic Ocean, and extends 110 miles southwestward across Peninsular Florida into deep water in the Gulf of Mexico near Yankeetown. The canal will be equipped with five locks, each 600 feet long and 84 feet wide, and the channel will be a minimum of 12 feet deep and 150 feet wide. From near Ocala northeastward, the canal channel will replace much of the natural channel of the Oklawaha River, and will be excavated into beds of the so-called shallow sand aquifer of Miocene age and younger, which overlies limestone of the Floridan aquifer. Westward from Ocala, most of the canal will be excavated below the potentiometric surface into limestone and dolomite of the Floridan aquifer. Water levels of Rodman, Eureka, and Inglis Pools will be controlled by dams and spillways with the limited exchange of water between the pools and the aquifers. The water levels in the Summit Pools will fluctuate with the natural changes in the ground-water level of the Floridan aquifer, although the stage of the pool will be controlled partly by the stage held in the Eureka Pool. A dynamic inflow-outflow relationship will exist between the Summit Pool and the Floridan aquifer. The Floridan aquifer in the canal area is 1,000 to 1,200 feet thick and consists of limestone and dolomite of middle Eocene Miocene age, including from older to younger, the Lake City, Avon Park, and Ocala limestones plus permeable sandy, dolomitic limestone in the lower part of the Hawthorn Formation. It is possible that most of the flow to the two major springs in the area occurs in the upper 100 feet or so of the aquifer in the Ocala Limestone. The aquifer is underlain by the Oldsmar limestone of early Eocene age and is overlain by sand, clayey sand, clay and shell beds of Miocene through Holocene age, in thickness from a few feet to 300 feet. The permeable beds overlying the Floridan aquifer constitute the shallow aquifer, while the poorly permeable ones act as confining beds where the Floridan aquifer is under artesian conditions. A north-south line drawn separating the head of Silver Springs on the west from the Oklawaha River on the east marks the approximate western limit of a continuous blanket of materials of Miocene-Pliocene(?) age covering the rocks of the Floridan aquifer. East of the line, much of the aquifer is under artesian conditions, particularly in the Oklawaha River valley, although in some areas east of the valley, direct recharge through thick permeable Miocene-Pliocene(?) sands occurs. West of the line, only scattered remnants of a once continuous Miocene-Pliocene(?) cover remain. Lack of the cover is a result of erosion on the crest and flank of the Ocala Uplift, a broad northwest-southeast trending anticlinal upwarp, the axis of which is crossed by the canal route in the Dunnellon area. Over most of this area the Floridan aquifer is unconfined and receives direct recharge through a cover of a few tens of feet of sand and clayey sand of Quaternary age. Tensional stresses during the structural evolution of the Ocala Uplift produced an intersecting system of fractures and normal faults in rocks of the Floridan aquifer. The fractures and faults are important controls for orientation of solution channels and, therefore, for development of ground-water circulation patterns. When the system surface streams, which once drained the Barge Canal area, eroded the poorly permeable Miocene-Pliocene(?) cover from the flanks of the Ocala Uplift, surface runoff was reduced and precipitation began to directly infiltrate the underlying limestones. Now only principal streams remain, such as the Oklawaha and Withlacoochee Rivers and a few short tributaries, while one of the most highly developed subsurface drainage systems in the world has evolved in cavernous limestone of the Floridan aquifer. Two of the larger freshwater springs in the world now discharge from the Floridan aquifer in the canal area. Silver Springs near Ocala discharges an average 531 mgd (million gallons per day) down the 4-mile long Silver River, which flows on poorly permeable beds to the Oklawaha River. Rainbow Springs near Dunnellon discharges on average 468 mgd from numerous orifices in the bed of the 5-mile-long Rainbow River, which flows into the Withlacoochee River. The heads of the springs have migrated to their present positions partly because of a tendency of ground-water levels to decline as permeability in the aquifer is increased due to removal of limestone by solution, and because of mechanical erosion of the limestone in the vicinity of the spring heads. Also, points of principal spring discharges have shifted in the past due to changes in ground-water levels in response to changes in sea level. The subsurface drainage system is continuing to evolve today, as evidenced in part by frequent occurrence of new sinkholes and by the presence of significant amounts of calcium bicarbonate in the spring waters. Rodman Pool, at the east end of the canal, is separated from the Floridan aquifer by poorly permeable materials. The pool's operating water level will be only a few feet above the potentiometric surface at the downstream end, and at or slightly below the potentiometric surface at the upstream end. Little exchange of water between the Rodman Pool and the Floridan aquifer is expected. Eureka Pool, just upstream from Rodman Pool, will also be separated from the Floridan aquifer by poorly permeable beds. However, the stage of the pool will be about 15 feet higher than the natural potentiometric surface at the pool's downstream end, and some seepage into the Floridan aquifer is anticipated through faults and leaky parts of the poorly permeable beds, with a consequent rise in ground-water levels in areas adjacent to the lower end of the pool. Possibilities for particulate contamination of the aquifer will tend to be minimized because of the filtering capacity of the materials through which water must pass to reach the aquifer, although the natural filter will not preclude movement into the aquifer of contaminants which might become dissolved in the pool waters. No significant interchange of water between the pool and the aquifer is expected at the upstream end of the Eureka Pool. Present construction plans indicate an operating stage for Eureka Pool which will range between 38 and 40 feet above mean sea level, although it is possible to dredge the pool deep enough to permit a range in stage of 36 to 40 feet. A backwater effect extending up Silver River from the Eureka Pool is expected to regulate the stage at the head of Silver Springs between 39 and 44 feet above mean sea level if the pool ranges between 36 and 40 feet. If Eureka Pool ranges only between 38 and 40 feet, the range of stage at the head of the springs should be about 41 to 44 feet above mean sea level. From Inglis Lock west, the canal will have direct connection with the Gulf, and canal stage will fluctuate with the Gulf tide. Since the canal stage will be slightly lower than the adjacent ground-water levels along much of the reach, there will be some ground-water inflow to the canal. No significant changes in the existing ground-water regime are expected in the vicinity of Inglis Pool, the first step up in the canal east of the Gulf. Existing ground-water and surface-water levels in the area will not change appreciable, and the natural stage and flow of Rainbow Springs, which will flow by way of Rainbow River into Inglis Pool, should not be affected by canal operations. A possible adverse effect of the canal on the Inglis Pool area could result if sea water is locked up from the Gulf through Inglis Lock. However, the high step of 25 feet at the lock, flushing action of continuous flow from Inglis Pool to the lower reaches of the Withlacoochee River, and use of possible preventive locking procedures should minimize the problem. The potential for adverse effects on the ground-water regime is greatest in the area of the Summit Pool. Through most of the length of the pool, the canal channel will be excavated into limestone of the Floridan aquifer to depths of 12 to 27 feet below the potentiometric surface. Changes that will take place in the ground-water flow system in the Silver Springs drainage area, once the canal is completed, were estimated by flow-net analysis. Variation in aquifer transmissivity was determined by calculating transmissivity in 25 different flow cells surrounding Silver Springs. Transmissivity in the 25 cells averages about 15,600,000 gpd/ft [2,090,000 ft2/day (feet squared)], but transmissivity in the six cells through which the Summit Pool passes ranges from 9,000,000 to 44,000,000 gpd/ft (1,210,000 to 5,900,000 ft2/day). Transmissivity was used to compute static stage of the Summit Pool under given ground-water level conditions. Had the canal existed in May 1968 and had the stage of Eureka Pool been held at 36 feet at the time, the static stage in Summit Pool would have been about 42.1 feet above mean sea level. Thus, a conceptual model of the changes in the potentiometric surface wrought by the finished canal was drawn, and zones of ground-water inflow and outflow were delineated. Most outflow from the Summit Pool to the aquifer should be limited to one 4-mile-long zone along the north side of the pool, about 5 miles south of Silver Springs. It is estimated that a water volume equivalent to about 8 percent of the daily flow of Silver Springs will enter the Summit Pool each day from the southern one-third of the Silver Springs drainage area. A like amount will reenter the aquifer at the main zone of outflow and move toward Silver Springs at an estimated average velocity of about 200 feet per day, if something close to the natural static stage of the pool is maintained by return pumpage of the lockage losses. At a velocity of 200 feet per day, water from the Summit Pool would discharge at Silver Springs about 140 days later. However, any estimate of velocity in the highly cavernous limestone aquifer in the area should be used with caution, because difficult to measure changes in porosity and thickness of major zones of flow may cause large variations in velocity. If all lockage losses are returned to the Summit Pool by pumping from Eureka Pool, no net loss from the Silver Springs drainage area, except for some evaporation from the water surface in the canal and possible leakage around locks, will result from canal operations. The zone of outflow from the Summit Pool to the aquifer will be in a natural potentiometric trough, and the zone of inflow will be in a potentiometric ridge area. The equilibrium water level in the pool will tend to be about 1 foot higher than the altitude of the lowest level in the pre-canal potentiometric trough, and about 2 feet lower than the highest level on the pre-canal potentiometric ridge. West of the Silver Springs drainage area just east of Dunnellon Lock, in the area of a local potentiometric high, the water level in the Summit Pool is expected to be about 15 feet below the natural potentiometric surface. In most areas 2 to 3 miles away from the Summit Pool, effects of the canal on the natural potentiometric surface should be slight. The stage of the Summit Pool, judging from the 36-year record for ground-water level changes and the anticipated indirect effect of the controlled stage in Eureka Pool, should have a maximum of about 10.5 feet with a maximum water level of about 51.5 feet above mean sea level and a minimum of about 41.0 feet above mean sea level. Of particular importance in the Summit Pool is an implementation of well planned construction and operational procedures designed to minimize risks of ground-water contamination.

Florida↗