Geology topics

JoAnn M. Gronberg

Publications and source records attributed to JoAnn M. Gronberg.

14 recordsLinked to original sources

USGS2021

Precipitation-runoff processes in the Merced River Basin, Central California, with prospects for streamflow predictability, water years 1952–2013

The U.S. Geological Survey, in cooperation with the California Department of Water Resources (DWR), has constructed a new spatially detailed Precipitation-Runoff Modeling System (PRMS) model for the Merced River Basin, California, which is a tributary of the San Joaquin River in California. Operated through an Object User Interface (OUI) with Ensemble Streamflow Prediction (ESP) and daily climate distribution preprocessing functionality, the model is calibrated primarily to simulate (and eventually, forecast) year-to-year variations of inflows to Lake McClure during the critical April–July snowmelt season. The model is intended to become part of a suite of methods used by DWR for estimating daily streamflow from the Merced River Basin, especially during the snowmelt season. This study describes the results of the application of an analysis tool that simulates responses to climate and land-use variations at a higher spatial resolution than previously available to DWR. A geographic information system was used to delineate the model domain, that is, areas draining to a single outlet at U.S. Geological Survey streamflow-gaging station 11270900, Merced River below Merced Falls Dam, near Snell, CA (also known as California Data Exchange Center station MRC), and subdrainage areas, including four draining to internal gages used as calibration targets. Using this delineation, three contiguous subbasins were recognized and, along with the model domain and nested calibration targets, are the simulation units evaluated in this report. An auto-calibration tool, LUCA (Let Us CAlibrate), was used for each calibration node, from headwaters to basin outlet, and then parameters were manually adjusted to complete the calibration. The main objective was to match April–July snowmelt seasonal discharge values of simulated streamflow to observed (measured or reconstructed) discharge values. Calibration or validation periods used site-specific streamflows—mostly from October 1, 1988, through September 30, 2013—but differed according to the period-of-record available for the measurements collected at internal gages or reconstructed flows for the single outlet. The accuracy of the Merced PRMS streamflow simulations varied seasonally, as compared to observed values. Based on statistical results, the Merced PRMS model satisfactorily simulated snowmelt seasonal streamflows. April–July calibrations for all areas had small negative bias (not greater than 7 percent) and low relative error (less than 8 percent). Less satisfactory performance for other seasons was attributed to several factors: (1) high uncertainty in low or zero flows in summer and fall, (2) lack of accounting for basin withdrawals and anthropogenic water use, (3) unavailability and (or) inaccuracy of observed (measured) meteorological input data, and (4) uncertainty in reconstructed streamflow data. With some additional refinement, the Merced PRMS model may be used for forecasting seasonal and longer-term streamflow variations; evaluating forecasted and past climate and land cover changes; providing water-resource managers with a consistent and documented method for estimating streamflow at ungaged sites within the basin; and aiding environmental studies, hydraulic design, water management, and water-quality projects in the Merced River Basin.

California↗

USGS2018

Maps showing predicted probabilities for selected dissolved oxygen and dissolved manganese threshold events in depth zones used by the domestic and public drinking water supply wells, Central Valley, California

The purpose of the prediction grids for selected redox constituents—dissolved oxygen and dissolved manganese—are intended to provide an understanding of groundwater-quality conditions at the domestic and public-supply drinking water depths. The chemical quality of groundwater and the fate of many contaminants is influenced by redox processes in all aquifers, and understanding the redox conditions horizontally and vertically is critical in evaluating groundwater quality. The redox condition of groundwater—whether oxic (oxygen present) or anoxic (oxygen absent)—strongly influences the oxidation state of a chemical in groundwater. The anoxic dissolved oxygen thresholds of <0.5 milligram per liter (mg/L), <1.0 mg/L, and <2.0 mg/L were selected to apply broadly to regional groundwater-quality investigations. Although the presence of dissolved manganese in groundwater indicates strongly reducing (anoxic) groundwater conditions, it is also considered a “nuisance” constituent in drinking water, making drinking water undesirable with respect to taste, staining, or scaling. Three dissolved manganese thresholds, <50 micrograms per liter (µg/L), <150 µg/L, and <300 µg/L, were selected to create predicted probabilities of exceedances in depth zones used by domestic and public-supply water wells. The 50 µg/L event threshold represents the secondary maximum contaminant level (SMCL) benchmark for manganese (U.S. Environmental Protection Agency, 2017; California Division of Drinking Water, 2014), whereas the 300 µg/L event threshold represents the U.S. Geological Survey (USGS) health-based screening level (HBSL) benchmark, used to put measured concentrations of drinking-water contaminants into a human-health context (Toccalino and others, 2014). The 150 µg/L event threshold represents one-half the USGS HBSL. The resultant dissolved oxygen and dissolved manganese prediction grids may be of interest to water-resource managers, water-quality researchers, and groundwater modelers concerned with the occurrence of natural and anthropogenic contaminants related to anoxic conditions. Prediction grids for selected redox constituents and thresholds were created by the USGS National Water-Quality Assessment (NAWQA) modeling and mapping team.

California↗

USGS2017

County-level estimates of nitrogen and phosphorus from animal manure for the conterminous United States, 2007 and 2012

County-level estimates of nitrogen and phosphorus inputs from animal manure for the conterminous United States were calculated from animal population inventories in the 2007 and 2012 Census of Agriculture, using previously published methods. These estimates of non-point nitrogen and phosphorus inputs from animal manure were compiled in support of the U.S. Geological Survey’s National Water-Quality Assessment Project of the National Water Quality Program and are needed to support national-scale investigations of stream and groundwater water quality. The estimates published in this report are comparable with older estimates which can be compared to show changes in nitrogen and phosphorus inputs from manure over time.

Open-File Report↗

USGS2017

Prediction and visualization of redox conditions in the groundwater of Central Valley, California

Regional-scale, three-dimensional continuous probability models, were constructed for aspects of redox conditions in the groundwater system of the Central Valley, California. These models yield grids depicting the probability that groundwater in a particular location will have dissolved oxygen (DO) concentrations less than selected threshold values representing anoxic groundwater conditions, or will have dissolved manganese (Mn) concentrations greater than selected threshold values representing secondary drinking water-quality contaminant levels (SMCL) and health-based screening levels (HBSL). The probability models were constrained by the alluvial boundary of the Central Valley to a depth of approximately 300 m. Probability distribution grids can be extracted from the 3-D models at any desired depth, and are of interest to water-resource managers, water-quality researchers, and groundwater modelers concerned with the occurrence of natural and anthropogenic contaminants related to anoxic conditions. Models were constructed using a Boosted Regression Trees (BRT) machine learning technique that produces many trees as part of an additive model and has the ability to handle many variables, automatically incorporate interactions, and is resistant to collinearity. Machine learning methods for statistical prediction are becoming increasing popular in that they do not require assumptions associated with traditional hypothesis testing. Models were constructed using measured dissolved oxygen and manganese concentrations sampled from 2767 wells within the alluvial boundary of the Central Valley, and over 60 explanatory variables representing regional-scale soil properties, soil chemistry, land use, aquifer textures, and aquifer hydrologic properties. Models were trained on a USGS dataset of 932 wells, and evaluated on an independent hold-out dataset of 1835 wells from the California Division of Drinking Water. We used cross-validation to assess the predictive performance of models of varying complexity, as a basis for selecting final models. Trained models were applied to cross-validation testing data and a separate hold-out dataset to evaluate model predictive performance by emphasizing three model metrics of fit: Kappa; accuracy; and the area under the receiver operator characteristic curve (ROC). The final trained models were used for mapping predictions at discrete depths to a depth of 304.8 m. Trained DO and Mn models had accuracies of 86–100%, Kappa values of 0.69–0.99, and ROC values of 0.92–1.0. Model accuracies for cross-validation testing datasets were 82–95% and ROC values were 0.87–0.91, indicating good predictive performance. Kappas for the cross-validation testing dataset were 0.30–0.69, indicating fair to substantial agreement between testing observations and model predictions. Hold-out data were available for the manganese model only and indicated accuracies of 89–97%, ROC values of 0.73–0.75, and Kappa values of 0.06–0.30. The predictive performance of both the DO and Mn models was reasonable, considering all three of these fit metrics and the low percentages of low-DO and high-Mn events in the data.

California↗

USGS2016

Predicting arsenic in drinking water wells of the Central Valley, California

Probabilities of arsenic in groundwater at depths used for domestic and public supply in the Central Valley of California are predicted using weak-learner ensemble models (boosted regression trees, BRT) and more traditional linear models (logistic regression, LR). Both methods captured major processes that affect arsenic concentrations, such as the chemical evolution of groundwater, redox differences, and the influence of aquifer geochemistry. Inferred flow-path length was the most important variable but near-surface-aquifer geochemical data also were significant. A unique feature of this study was that previously predicted nitrate concentrations in three dimensions were themselves predictive of arsenic and indicated an important redox effect at >10 μg/L, indicating low arsenic where nitrate was high. Additionally, a variable representing three-dimensional aquifer texture from the Central Valley Hydrologic Model was an important predictor, indicating high arsenic associated with fine-grained aquifer sediment. BRT outperformed LR at the 5 μg/L threshold in all five predictive performance measures and at 10 μg/L in four out of five measures. BRT yielded higher prediction sensitivity (39%) than LR (18%) at the 10 μg/L threshold–a useful outcome because a major objective of the modeling was to improve our ability to predict high arsenic areas.

California↗

USGS2014

Modeling nitrate at domestic and public-supply well depths in the Central Valley, California

Aquifer vulnerability models were developed to map groundwater nitrate concentration at domestic and public-supply well depths in the Central Valley, California. We compared three modeling methods for ability to predict nitrate concentration >4 mg/L: logistic regression (LR), random forest classification (RFC), and random forest regression (RFR). All three models indicated processes of nitrogen fertilizer input at the land surface, transmission through coarse-textured, well-drained soils, and transport in the aquifer to the well screen. The total percent correct predictions were similar among the three models (69–82%), but RFR had greater sensitivity (84% for shallow wells and 51% for deep wells). The results suggest that RFR can better identify areas with high nitrate concentration but that LR and RFC may better describe bulk conditions in the aquifer. A unique aspect of the modeling approach was inclusion of outputs from previous, physically based hydrologic and textural models as predictor variables, which were important to the models. Vertical water fluxes in the aquifer and percent coarse material above the well screen were ranked moderately high-to-high in the RFR models, and the average vertical water flux during the irrigation season was highly significant (p < 0.0001) in logistic regression.

California↗

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

Development and Application of Regression Models for Estimating Nutrient Concentrations in Streams of the Conterminous United States, 1992-2001

Data collected for the U.S. Geological Survey National Water-Quality Assessment program from 1992-2001 were used to investigate the relations between nutrient concentrations and nutrient sources, hydrology, and basin characteristics. Regression models were developed to estimate annual flow-weighted concentrations of total nitrogen and total phosphorus using explanatory variables derived from currently available national ancillary data. Different total-nitrogen regression models were used for agricultural (25 percent or more of basin area classified as agricultural land use) and nonagricultural basins. Atmospheric, fertilizer, and manure inputs of nitrogen, percent sand in soil, subsurface drainage, overland flow, mean annual precipitation, and percent undeveloped area were significant variables in the agricultural basin total nitrogen model. Significant explanatory variables in the nonagricultural total nitrogen model were total nonpoint-source nitrogen input (sum of nitrogen from manure, fertilizer, and atmospheric deposition), population density, mean annual runoff, and percent base flow. The concentrations of nutrients derived from regression (CONDOR) models were applied to drainage basins associated with the U.S. Environmental Protection Agency (USEPA) River Reach File (RF1) to predict flow-weighted mean annual total nitrogen concentrations for the conterminous United States. The majority of stream miles in the Nation have predicted concentrations less than 5 milligrams per liter. Concentrations greater than 5 milligrams per liter were predicted for a broad area extending from Ohio to eastern Nebraska, areas spatially associated with greater application of fertilizer and manure. Probabilities that mean annual total-nitrogen concentrations exceed the USEPA regional nutrient criteria were determined by incorporating model prediction uncertainty. In all nutrient regions where criteria have been established, there is at least a 50 percent probability of exceeding the criteria in more than half of the stream miles. Dividing calibration sites into agricultural and nonagricultural groups did not improve the explanatory capability for total phosphorus models. The group of explanatory variables that yielded the lowest model error for mean annual total phosphorus concentrations includes phosphorus input from manure, population density, amounts of range land and forest land, percent sand in soil, and percent base flow. However, the large unexplained variability and associated model error precluded the use of the total phosphorus model for nationwide extrapolations.

Scientific Investigations Report↗

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↗

USGS2005

Data on dissolved pesticides and volatile organic compounds in surface and ground waters in the San Joaquin-Tulare basins, California, water years 1992-1995

This report contains pesticide, volatile organic compound, major ion, nutrient, tritium, stable isotope, organic carbon, and trace-metal data collected from 149 ground-water wells, and pesticide data collected from 39 surface-water stream sites in the San Joaquin Valley of California. Included with the ground-water data are field measurements of pH, specific conductance, alkalinity, temperature, and dissolved oxygen. This report describes data collection procedures, analytical methods, quality assurance, and quality controls used by the National Water-Quality Assessment Program to ensure data reliability. Data contained in this report were collected during a four year period by the San Joaquin?Tulare Basins Study Unit of the United States Geological Survey's National Water-Quality Assessment Program. Surface-water-quality data collection began in April 1992, with sampling done three times a week at three sites as part of a pilot study conducted to provide background information for the surface-water-study design. Monthly samples were collected at 10 sites for major ions and nutrients from January 1993 to March 1995. Additional samples were collected at four of these sites, from January to December 1993, to study spatial and temporal variability in dissolved pesticide concentrations. Samples for several synoptic studies were collected from 1993 to 1995. Ground-water-quality data collection was restricted to the eastern alluvial fans subarea of the San Joaquin Valley. Data collection began in 1993 with the sampling of 21 wells in vineyard land-use settings. In 1994, 29 wells were sampled in almond land-use settings and 9 in vineyard land-use settings; an additional 11 wells were sampled along a flow path in the eastern Fresno County vineyard land-use area. Among the 79 wells sampled in 1995, 30 wells were in the corn, alfalfa, and vegetable land-use setting, and 1 well was in the vineyard land-use setting; an additional 20 were flow-path wells. Also sampled in 1995 were 28 wells used for a regional assessment of ground-water quality in the eastern San Joaquin Valley.

Data Series↗

USGS2004

Occurrence, Distribution, Instantaneous Loads, and Yields of Dissolved Pesticides in the San Joaquin River Basin, California, During Summer Conditions, 1994 and 2001

Water samples were collected from 22 drainage basins for analysis of 48 dissolved pesticides during summer flow conditions in 1994 and 2001. Of the 48 pesticides, 31 were reported applied in the basin in the 28 days preceding the June 1994 sampling, 25 in the 28 days preceding the June 2001 sampling, and 24 in the 28 days preceding the August 2001 sampling. The number of dissolved pesticides detected was similar among sampling periods: 26 were detected in June 1994, 28 in June 2001, and 27 in August 2001. Concentrations of chlorpyrifos exceeded the California criterion for the protection of aquatic life from acute exposure at six sites in June 1994 and at five sites in June 2001. There was a single exceedance of the criterion for diazinon in June 1994. The number of pesticides applied in tributary basins was highly correlated with basin area during each sampling period (Spearman's r = 0.85, 0.70, and 0.84 in June 1994, June 2001, and August 2001, respectively, and p < 0.01 in all cases). Larger areas likely include a wider variety of crops, resulting in more varied pesticide use. Jaccard's similarities, cluster analysis, principal components analysis, and instantaneous load calculations generally indicate that west-side tributary basins were different from east-side tributary basins. In general, west-side basins had higher concentrations, instantaneous loads, and instantaneous yields of dissolved pesticides than east-side basins, although there were a number of exceptions. These differences may be related to a number of factors, including differences in basin size, soil texture, land use, irrigation practices, and stream discharge.

Scientific 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

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↗

USGS1994

Hydrogeology, simulation of regional ground-water flow, and saltwater intrusion, Potomac-Raritan-Magothy Aquifer System, Northern Coastal Plain of New Jersey

The Potomac-Raritan-Magothy aquifer system in Middlesex and Monmouth Counties in the northern Coastal Plain of New Jersey consists primarily of unconsolidated Cretaceous sediments, which are divided into the upper and middle aquifers and confining units. These units, which strike northeastsouthwest along the Fall Line, dip and thicken to the southeast. The upper aquifer consists primarily of the Old Bridge Sand Member of the Magothy Formation, which is composed of coarse-grained sands, localized thin clay beds, and younger surficial sands and gravels in and near the outcrop. Transmissivity ranges from 1,760 to 19,400 ft 2 /d (feet squared per day) and tends to be higher in updip areas. Estimated withdrawals from the upper aquifer in the northern Coastal Plain were approximately 42 Mgal/d (million gallons per day) in 1986. Cones of depression whose centers range from 36 to 42 ft (feet) below sea level have developed as a result of these withdrawals. The upper aquifer is confined throughout most of the northern New Jersey Coastal Plain by clays and silts of the Cretaceous Woodbury Clay and Merchantville Formation and younger sediments of the Magothy Formation. This confining unit generally is greater than 200 ft thick. The simulated vertical hydraulic conductivity for the confining unit ranges from 8.4 x 10 -5 to 5.6 x 10 -3 feet per day; interpreted vertical hydraulic conductivities generally are lower except in southwestern Middlesex County, where the vertical hydraulic conductivities of the confining unit are higher. The middle aquifer consists primarily of the Farrington Sand Member of the Cretaceous Raritan Formation and surficial Holocene and Miocene sands and gravels in its outcrop area. It also can include the uppermost sands of the Cretaceous Potomac Group in parts of Monmouth County. The middle aquifer is composed of fine to coarse sand that contains some lignite and pyrite, and, locally, some clay beds. It pinches out in the northern part of Sayreville Township, near Raritan River. The transmissivity of the aquifer ranges from 2,140 to 13,800 ft 2/ d and tends to decrease in the northern part of the northern Coastal Plain of New Jersey where the aquifer thins. A poorly permeable confining unit composed mostly of clays and silts of the Woodbridge Clay Member of the Raritan Formation overlies the aquifer in most of this area. The confining unit generally is greater than 100 ft thick, although it thins and is sandy in the southwestern part of Middlesex County, where a good hydraulic connection exists between the middle and upper aquifers. Estimated withdrawals from the middle aquifer in the northern Coastal Plain were about 22 Mgal/d in 1986. These withdrawals have caused cones of depression whose centers range from 77 to 93 ft below sea level. A finite-difference, quasi-three-dimensional ground-water flow model was developed to simulate ground-water flow in the aquifer system. The confined and unconfined areas of the upper and middle aquifers were modeled as separate layers. The model was calibrated primarily by adjusting vertical hydraulic conductivity in the confining units and horizontal hydraulic conductivity in the aquifers, then matching simulated and measured groundwater levels for the period 1896-1986 and simulated and interpreted potentiometric surfaces under predevelopment conditions and in 1984. For the predevelopment period, the total flow into and out of the upper and middle aquifers is 35 and 21 Mgal/d, respectively. Recharge to the aquifer system is from direct recharge in the unconfined areas and from vertical leakage through overlying confining units. The main recharge areas are the topographically high areas in southwestern Middlesex County for both aquifers, in the eastern Sayreville area for the upper aquifer, and north of the Raritan River for the middle aquifer. Most ground water discharges to low-lying regional surface-water drains (streams), which flow into the South River. For 1984 transient conditions, the total ground-water flow into and out of the upper and middle aquifers is 61 and 34 Mgal/d, respectively. The largest amount of recharge is from direct recharge in the unconfined areas, but some recharge also is derived from vertical leakage through the Merchantville-Woodbury confining unit, captured ground-water discharge to streams, and induced inflow at artificial-recharge facilities. Regional flow is from recharge areas toward major cones of depression. Sensitivity analysis showed that the model was useful for representing flow in the system, especially in the confined-aquifer areas. Model representation of lateral and vertical boundary conditions was judged acceptable. Simulation results were less sensitive to changes in aquifer properties in the unconfined areas of the aquifers and to changes in storage in the confining units. Sensitivity analysis and calibration of hydraulic parameters and conditions showed that the distribution of hydraulic head was sensitive to changes in horizontal hydraulic conductivity in the aquifers, vertical hydraulic conductivity in the confining units, magnitudes of ground-water withdrawals, and initial hydraulic head in aquifer outcrop areas. Two scenarios were simulated to determine the effects of ground-water withdrawals from 1986 through 2019. For the scenario in which ground-water withdrawals increase to about 69 Mgal/d in the upper aquifer and 37 Mgal/d in the middle aquifer, centers of cones of depression are as deep as 100 ft below sea level in the upper aquifer and 170 ft below sea level in the middle aquifer. For this scenario, most of the additional water comes from captured surface-water discharge, induced cross-formational flow from overlying aquifers, and increases in induced flow from artificial-recharge areas. Induced flow from Raritan Bay also increases. For the scenario in which ground water withdrawals are reduced to 42.5 Mgal/d in the upper aquifer and 15 Mgal/d in the middle aquifer, water levels recover to above sea level nearly everywhere. In each aquifer, ground-water discharge to streams increases and induced flow through the confining units and from the overlying sediments decreases, and discharge of ground water to Raritan Bay in the upper aquifer exceeds the induced recharge from Raritan Bay. Reversal of ground-water gradients has caused saltwater intrusion in the two aquifers. Chloride concentrations in water from the upper aquifer in Keyport and Union Beach Boroughs were as high as 2,100 mg/L (milligrams per liter) in 1986. The intrusion has not increased significantly since well fields in the area were closed in the late 1970's. Elevated chloride concentrations also were measured in Keanesburg Borough in 1986. In both of these areas, saltwater has entered the upper aquifer from the Bay because of movement of the freshwater-saltwater interface in response to increasing ground-water withdrawals. Chloride concentrations in well-water samples from the middle aquifer were as high as 6,000 mg/L in Sayreville Borough in 1987; concentrations in samples from drive-point wells from the same aquifer near the Washington Canal, the main source of saltwater, were as high as 7,100 mg/L. The migration of the saltwater front at about 470 feet per year to the southeast is influenced mainly by a thinning of the middle aquifer, which constrains flow, and by the locations of regional cones of depression caused by groundwater withdrawals.

New Jersey↗