LINJ 1998 Groundwater

Science Center Objects

The water quality in aquifer systems of the LINJ has been extensively investigated. The areal extent and number of these investigations, however, varies across the study unit. The fractured bedrock and valley-fill aquifers of the Piedmont and New England provinces in northern NJ have been the subject of fewer and more site-specific investigations, whereas, the unconsolidated sand and gravel aquifers of the Coastal Plain of LI and NJ have been the subject of numerous site-specific and regional water-quality investigations. Consequently, more is known about the geohydrology and water quality of the Coastal Plain aquifers.

Background and Perspective

Numerous groundwater quality investigations undertaken as part of the Toxic Substances Hydrology Program in the Coastal Plain examined relations between land use and shallow groundwater quality in regionally extensive areas of LI and NJ (Barton and others, 1987; Eckhardt and others, 1989; Stackelberg, 1995; Eckhardt and Stackelberg, 1995; and Vowinkel and Battaglin, 1995). Eckhardt and others (1989) evaluated the relation between land use and groundwater quality in the surficial aquifer system of Nassau and Suffolk Counties on LI examining over 14,000 chemical analyses of samples from 903 wells collected between 1978-84. Results indicate that contamination from human activities has affected water quality in the surficial aquifer system. The occurrence of volatile organic compounds and pesticides was confirmed and statistically significant correlations between land use and these and other contaminants were established.

In a later investigation, Eckhardt and Stackelberg (1995), water-quality data from 90 monitoring wells screened near the water table beneath five areas of differing land use were compared. Results indicate that samples from undeveloped areas had the lowest and smallest range in concentrations of most human-derived constituents such as nitrate, boron, VOCs, and pesticides. Concentrations of these constituents in samples from three suburban areas and an agricultural area generally were intermediate to high and had the widest variation. Equations predicting the occurrence of contaminants near the water table were developed using logistic regression analyses of explanatory variables that characterize the type of land use and population density within a 1/4-mile radius of each of the 90 wells.

Vowinkel and Battaglin (in press) evaluated the effects of nonpoint sources of contamination on the quality of water in aquifers of the Coastal Plain in New Jersey. Water-quality data from over 1,000 wells sampled between 1980-89 for major ions, nutrients, trace elements, and organic constituents including dissolved organic carbon, phenols, volatile organic compounds, and pesticides were examined. Results indicate that nonpoint sources of contamination significantly affect the quality of shallow groundwater in aquifers of the Coastal Plain in New Jersey and that the distribution of contaminants are significantly related to patterns of land use.

The vulnerability of water withdrawn from public-supply wells to contamination by pesticides and volatile organic compounds has also been statistically and deterministically evaluated for aquifers in the Coastal Plain (Navoy, 1993; Vowinkel and others, 1994). Navoy (1993) utilized a finely discretized groundwater-flow model and flow-path simulation to demonstrate the vulnerability of Coastal Plain aquifers to non-point sources of contamination. Navoy (1993) applied a quantitative understanding of the groundwater flow system of the Potomac-Raritan-Magothy aquifer system in Gloucester County, NJ, to identify public-supply wells where contamination by VOCs was likely. Results from this analysis were consistent with available water-quality data and indicate that wells currently unaffected by contamination probably will be affected by contamination in the future and that the concentrations of VOCs in water from affected wells is likely to increase. The method of investigation utilized by Navoy (1993) demonstrates the (1) utility of a quantitative approach to understanding water-quality issues in water-supply aquifers, and (2) transferability of such methods to other locales where a finely discretized groundwater flow model and high-resolution land-use data are available.

Beginning in 1995, Safe Drinking Water Act regulations required the 626 large community water systems in New Jersey to monitor their 2,600 wells quarterly for 23 pesticides. As part of a 3-year study that began in October 1992, the USGS, in cooperation with the New Jersey Department of Environmental Protection (NJDEP), developed a geographic information system (GIS) data base to provide data on the vulnerability of water from public supply wells to contamination by pesticides (Vowinkel and others, 1994). Vulnerability was determined by using a numerical rating method based on information in the GIS data base. The information will be used by the State to determine the level of monitoring as a function of the vulnerability rating and to give waivers if a well is not vulnerable. The vulnerability of a well to contamination by pesticides is defined by (1) the sensitivity of the aquifer to contamination and (2) the intensity of pesticide use in areas where the aquifer is sensitive. Three variables were used to predict aquifer sensitivity: (1) location of a well relative to the outcrop area, (2) soil organic matter content, and (3) depth from the land surface to the top of the open interval of the well (top of screen for wells in unconsolidated sediments and top of the open hole for bedrock aquifers). Three variables were used to predict pesticide-use intensity near wells that are sensitive to contamination: (1) predominant land use near the well, (2) distance from the nearest agricultural area, and (3) distance from the nearest golf course.

Where information was available, well-construction characteristics and location were determined for 2,100 of the 2,600 public supply wells in New Jersey. These wells are located in three different aquifer types: (1) Coastal Plain unconsolidated sand and gravel deposits, (2) unconsolidated glacial-deposit sediments, and (3) fractured bedrock. Using the numerical rating method, each well was assigned to one of 9 vulnerability groups on the basis of its sensitivity and intensity ratings. It was determined that about 26 percent of all public supply wells are not vulnerable to contamination from human activities. All wells in this low vulnerability group are in the confined parts of Coastal Plain aquifers. About 4 percent of public supply wells are ranked in the high sensitivity and high intensity group. These wells belong to the high vulnerability group, are located within the unconfined parts of aquifers and are within or adjacent to agricultural land. The remaining 70 percent of wells were determined to be moderately vulnerable to contamination from human activities and are largely unconfined and within or adjacent to residential or agricultural land.

Predicted model results were validated by analyzing water samples for pesticides and nitrate from a subset of 90 public supply wells throughout New Jersey. Wells were chosen from each combination of vulnerability category and aquifer category. Pesticides were detected in 6 of the 90 wells sampled. Three of these wells were rated in the high sensitivity and high intensity group. The other three wells were rated as having either high sensitivity or high intensity. None of the six wells were rated as having low sensitivity to contamination by pesticides.

Groundwater studies like those for predicting nitrate and VOC contamination of groundwater on Long Island using regression analysis and for predicting pesticide contamination of groundwater in New Jersey using the vulnerability rating method benefit everyone. The NJDEP estimated that monitoring waivers for pesticides granted for wells will save taxpayers almost $5 million annually for a one-time cost of $0.7 million. In addition, consulting firms, other Federal, State, and county agencies, universities and the general public make requests daily for the water-use, water-quality, and hydrogeologic data resulting from these and similar studies.

Groundwater Activities

The chemical quality of water in aquifer systems in most of the LINJ study unit has been extensively investigated. The fractured bedrock and valley-fill aquifers of the Piedmont and New England provinces in northern NJ have been the subject of fewer and more site-specific investigations, whereas, the unconsolidated sand and gravel aquifers of the Coastal Plain of LI and NJ have been the subject of numerous site-specific and regional water-quality investigations. Consequently, more is known about the Coastal Plain aquifers. The LINJ liaison committee identified a need to develop a better understanding of (1) relations between shallow groundwater quality and land-use patterns, (2) spatial and temporal trends in groundwater quality, (3) well contamination threat or vulnerability, (4) age-dating of groundwater, and (5) groundwater and surface-water interactions.

Our liaison committee recognized the discrepancy in data between the Coastal Plain and bedrock/valley-fill systems, but strongly recommended we concentrate our efforts on the analysis of existing data and towards collecting data that will allow us to further characterize source, transport, and fate issues for select contaminants throughout our study unit. Therefore, the groundwater component of the LINJ study unit has focused study-unit surveys in the less intensely studied bedrock aquifers of northern NJ and urban land-use and flow-path/chemical-transport surveys in the Coastal Plain of NJ. In addition to these core NAWQA activities, a more comprehensive characterization of the occurrence of contaminants in all relevant compartments of the hydrologic cycle is described in detail in the Comprehensive Urban Study workplan.
To date the LINJ team has installed 106 wells to accomplish various land-use and flow-path surveys. Seventy-two wells were installed and sampled during FY96-97 as part of an extended land-use survey (described in detail below). An additional 30 wells were installed in FY97 and will be sampled in FY98 as part of our regionally focused flow-path survey. One study-unit survey is currently underway and, if budget permits, a second study-unit survey is planned for FY98. This section describes our accomplishments to date and introduces the rational for our proposed groundwater activities in FY98.

HIP PLANS AS MODIFIED BY CONFERENCE CALLS

The present design for the LINJ GW effort encompasses:
(1) an extended LUS in the Glassboro study area (FY96-97)
(2) a regionally focused FPS in the Glassboro study area (FY96-98),
(3) a SUS of the NE fractured-bedrock aquifer systems in northern NJ (FY97-98)
(4) a SUS of the Kirkwood-Cohansey aquifer system in the Coastal Plain of NJ (FY98)
(5) a water-supply relevant investigation of aquifer vulnerability on LI (FY99), and
(6) retrospective SUS (FY 98-99) of areas not covered above in as much as possible.

It is not anticipated that all items identified above will be accomplished by the end of FY98 (Figure GW activities). The likelihood and scope for those items proposed for FY98 will be determined pending the completion of FY97 activities and the evaluation of data collected by GW and SW efforts during FY97.

The LINJ GW stratification (Figure Stratification)is detailed in the Environmental Setting section. Note that from the beginning we have considered the bedrock aquifers of northern NJ as a single strata. However, we were recently told not to combine the New England and Piedmont bedrock aquifers. Hence, our SUS now is for the New England fractured-bedrock aquifers only.

STUDY UNIT SURVEYS

Subunit surveys are to be conducted in areas that have limited groundwater quality data relative to other parts of the study unit, where drilling new wells is not practical, and where groundwater provides an important source of potable water. Primary objectives for study-unit surveys are to (1) provide a broad assessment of groundwater quality conditions in regionally extensive aquifers utilized for public and domestic supply, (2) identify those constituents causing the most prevalent water-quality concerns, and (3) to the extent possible, describe the occurrence and distribution of select constituents in relation to land-use patterns, geohydrology, soil types, and other natural and anthropogenic factors. The need to broadly assess water-quality conditions in major hydrogeologic units varies among study units depending on the availability and quality of existing data and the size and complexity of the groundwater systems (Gilliom and others, 1995).

Available water-quality data for the primary LINJ groundwater subunits have been compiled and screened to prioritize subunits in most need of study-unit surveys and to identify subunits in which analysis of existing data or sampling of existing wells may accomplish study-unit and/or land-use survey objectives. This section briefly describes the distribution of available water-quality data, the rational behind a study-unit survey currently being underway, and proposed plans for two additional study-unit surveys in FY98.

Accomplishments in FY96-97

Substantial amounts of water-quality data exist for groundwater subunits in the Coastal Plain of the LINJ. The relation between groundwater quality and land use was the focus of investigations undertaken in this province as part of the Toxic Substances Hydrology Program during the 1980's and early 1990's. Results from these and other investigations have been summarized in a retrospective publication tentatively entitled `How are people affecting groundwater quality in New Jersey and Long Island' by Clawges and others (through technical reviews; in editorial review). The scope and major findings from investigations of regional water quality in the Coastal Plain are summarized in the GW Background section and Previous Iinvestigations section of this workplan.

Considerably less regional-scale water-quality data exists for other groundwater subunits in the LINJ study area Table GW1. This is especially true for the valley-fill and fractured-bedrock aquifer systems of northern NJ (NE NJ) which are utilized extensively for domestic and public water supply. Based on liaison input, review of existing literature, and available water-quality data, the LINJ team has given the valley-fill and fractured-bedrock aquifer systems of northern New Jersey the highest priorities in terms of need for study-unit surveys.

In our FY97 workplan the LINJ team proposed a subunit survey of the valley-fill aquifer systems of northern NJ due to a relative scarcity of regional-scale water-quality data Table GW1, a regionally significant use of these systems for public water supply, and a highly vulnerable hydrogeologic setting. This survey was to be accomplished using existing observation wells screened within valley-fill sediments. During our FY97 conference call it was highly recommended that our subunit survey focus on that portion of the groundwater resource tapped by domestic wells. A subsequent retrieval from the NJDEP database of domestic-well permits issued since 1990 indicated that very few domestic wells installed since 1990 are screened within valley-fill sediments. Rather, the vast majority of domestic wells are being finished within the fractured bedrock systems of the New England province and the southern portion of the Piedmont province (Figure PHYSIO), not in the valley-fill systems. The northern Piedmont and valley-fill aquifers are largely developed for public water supply. Thus, the focus of our subunit survey has changed and now is designed to sample groundwater currently being tapped for domestic supply within the New England fractured bedrock systems (SUS-NJ NE). Thirty domestic wells finished in fractured bedrock within the New England province were randomly selected using a grid-based random site selection program (Scott, 1990).

Proposed SUS Work in FY98

Water-quality sampling of 30 subunit survey wells (SUS-NJ NE) in the New England fractured bedrock is currently underway and is expected to be completed during November 1997. Laboratory schedules included in this study-unit survey include field parameters, major ions and trace metals (SH2750), nutrients (SH2752), pesticides (SH2001 and SH2050), volatile organic compounds (SH2020), trace elements (LC2703), and dissolved radon gas. Quality-assurance samples will be collected at fifteen percent of the sites.

In a regional context, more SUS data exists (LSUS and POTO)or will likely be collected (DELR) for the Piedmont, the limited area of that strata in the LINJ currently being used for domestic supplies, and the lesser SUS data for the New England (CONN) and Coastal Plain (DELM) strata suggest a lower priority for the Piedmont. As a result of discussions with NST, a 30-well subunit survey of the Kirkwood-Cohansey aquifer system in the Coastal Plain of NJ (SUS-NJ CP) is also proposed for FY98. In addition to the reliance of this region on groundwater for public and domestic supply and the highly vulnerable setting of this aquifer system, there is a national/regional need to provide SUS-type data for this region of the Mid Atlantic Coastal Plain system.

The chemical quality of groundwater in the Coastal Plain of NJ is of particular concern because groundwater provides the majority of public and domestic supply. Withdrawals of groundwater for public supply alone average 268 Mgal/d; 176 Mgal/d from the confined PRM aquifer system and 54 Mgal/d from the surficial Kirkwood-Cohansey aquifer system (Nawyn and Clawges, 1995). Future withdrawals from portions of the PRM aquifer system have been restricted due to severe water-level declines and increased withdrawals from the surficial Kirkwood-Cohansey aquifer system is one alternative being ustilized to meet current and future water-supply demands. The surficial aquifer system is highly vulnerable to contamination introduced at or near land surface because it consists of highly permeable unconsolidated sands and gravels. The chemical quality of supplies from this system is of concern because of increasing withdrawals for public and domestic supply. In addition to providing valuable water-quality information for an important aquifer and needed data for this region of the Mid Atlantic Coastal Plain, this subunit survey will also provide a nested design to compare and contrast with data from the extended LUS and regional FPS of the Glassboro study area (described in detail in following sections).

This subunit survey will be accomplished by randomly selecting 30 domestic wells within the Kirkwood-Cohansey aquifer system of the NJ Coastal Plain. These wells will be sampled for the standard suite of SUS constituents (see above) plus dissolved mercury. Dissolved mercury is an issue in the CP but not in bedrock systems. Since we've already sampled 72 LUS wells and not found many hits in the SH2050 pesticide schedule, we would save considerable by not running this schedule for the CP SUS.

LAND USE SURVEYS

Primary objectives for land-use surveys are to (1) assess the concentrations and distribution of water-quality constituents in recently recharged groundwater associated with specific land-use settings, and (2) define the natural and anthropogenic factors associated with observed water-quality conditions in these land-use settings (Gilliom and others, 1995). This section describes a land-use survey accomplished in FY96-97 and the extension of this survey to address liaison committee concerns and to coordinate with the FPS and comprehensive urban investigation. A LUS was part of our FY96-97 activities, the LINJ team is not proposing any land-use surveys in FY98.

Accomplishments in FY96-97

The LINJ study unit encompasses some of the nations most densely populated and intensely developed regions. The last two decades have seen about a 10% increase in urban and suburban development; suburbanization is expected to continue in areas fringing major metropolitan centers. In the Philadelphia metropoiltan area, considerable growth is eastward into the southern portion of the NJ Coastal Plain.

Because of the wealth of available water-quality data and studies in the Coastal Plain and the liaison committee's desire to move beyond occurrence and distribution surveys, the LINJ team proposed a spatially focused, extended land-use survey which, when coordinated with the core FPS and the urban comprehensive project, is designed to increase the understanding of the sources and processes controlling the movement and fate of contaminants in groundwater systems in urban settings. The following discussion describes this survey.

Seventy-two shallow (approx 10 feet below WT) monitoring wells were located, installed, and sampled during summer and fall 1996 (FY96 and FY97) in the Glassboro region (Fig. GW-1) of the NJ CP. Well sites were distributed throughout all major land-use settings within the study area. Thirty wells were located in newly developed (<25 years) urban areas (lus new urban NJ1). The additional 42 wells (14 in older (>25 years) urban areas, 15 in agricultural areas, and 13 in undeveloped areas) were installed as part of the flow path study (fps ag/urban NJ1). This extension of the land-use survey provides water-quality data from other major land-use settings and allows for a more rigorous comparison of the effects of new urban land use. The LINJ liaison committee also recommended we identify sources of contaminants and processes which most significantly affect the transport and fate of select contaminants as they move through major compartments of the hydrologic cycle. Thus, a comprehensive urban investigation workplan was developed which will help address the major source, transport, and fate issues. The 72 well extended land-use survey represents an important component of the LINJ flow path study and comprehensive urban investigation by providing occurrence and distribution data at the water-table from all major land-use settings in our study area. This provides loading information to be used in the FPS modeling, which a 30-well urban LUS alone would not have provided. The 72 well network constitutes our initial efforts at a regional-scale FPS as described in the next section. The background and initial water-quality results for the 72 well survey is as follows.

The study area depicted in was targeted for an extended land-use survey for multiple reasons. The study area falls within the NJ State's Critical Water Supply Area #2 where serious water-supply problems have been identified by the NJ Department of Environmental Protection (NJDEP). Due to severe water-level declines in the confined Potomac-Raritan-Magothy aquifer system in this area the NJDEP has restricted further withdrawals from this aquifer system and approved increased withdrawals from the surficial Kirkwood-Cohansey aquifer system, thereby placing additional stress on the surficial system to meet current and future water-supply demands. Currently, there are over 250,000 residents within the urban land-use study area and population values are expected to increase 20% by the year 2010 due to anticipated suburban development (NJDEP, 1993). The surficial aquifer system in this area is comprised of highly permeable unconsolidated sands and gravels; thus, is highly vulnerable to contaminants introduced at or near land surface. The study area is also within the Philadelphia metropolitan area which is an EPA non-attainment region with respect to air quality, therefore, gasoline oxygenated with MTBE has been used year-round as mandated by the Clean Air Act Amendments of 1990.

Our new urban land-use survey consists of 30 wells and was designed following guidelines established by Squillace and Price (1996), as were the 42 wells in other land uses. Installation and sampling of all 72 wells in the extended LUS was accomplished using protocols and procedures described in Lapham and others (1995) and Koterba and others (1995). Potential well sites within each land-use setting were randomly selected using a grid-based random site selection program (Scott, 1990). Each well is constructed of 2-in. diameter PVC and was installed by hollow-stem auger. Wells were generally screened over a 2-foot interval about 10 feet below the water table. Continuous split-spoon samples of the unsaturated zone were collected during well installation at 20 new-urban sites and all of the old urban, agricultural, and undeveloped sites. These samples will be analyzed for sediment size, organic carbon content, soil moisture and pH, and the presence or absence of local finer-grained units. These characteristics are important because they may effect the movement of certain organic compounds through the unsaturated zone. All wells were sampled for the standard suite of chemical constituents identified by the NAWQA program including field parameters, major ions (SH2750), nutrients (SH2752), pesticides (SH2001/2050, SH1321, and SH0079), and volatile organic compounds (CM9090). Quality assurance samples were collected at 24% of the sites.

A summary of the major land-use settings targeted as part of this extended survey, the number of wells installed per land-use setting, and the number of wells sampled for various constituents. Figure GW-3 provides a summary of water-quality data.

Potential anthropogenic sources of contamination are being documented within a 500m radius of each well and stored in a GIS format to facilitate spatial analysis (LU/LC pilot). Data to be stored in GIS format include (1) the location of gas stations, dry cleaners, TRI sites, land fills, and other point sources, (2) local pipeline, sewer, and road networks, and (3) detailed representations of current land-use patterns and population density.
Results from the extended land-use survey have been summarized in a USGS Fact Sheet by Stackelberg and others (1997). This fact sheet is awaiting final approval and will be available for distribution shortly (early November 1997). A more rigourous interpretation of data collected as part of the extended land-use survey is currently underway and will result in a WRI and/or journal article during FY98.

Proposed LUS work in FY98

No new land-use surveys are currently planned, rather, we will proceed and conclude our GW sampling with the study-unit and flow-path surveys.

FLOW PATH SURVEYS

Primary objectives for flow-path surveys are to (1) characterize the spatial and temporal distributions of water quality in relation to groundwater flow in particular land-use settings, and (2) further the understanding of natural processes and human influences in these settings which affect the evolution of groundwater quality along flow paths (Gilliom and others, 1995). The primary liaison committee concern was to address the vulnerability of public and domestic water supplies to contamination from all relevant land-use settings. This section describes a regionally-focused flow-path survey designed to address NAWQA objectives and liaison committee concerns in an area with critical water-supply issues.

Accomplishments in FY96-97

Evaluating the source, transport, and fate of contaminants from a water-supply perspective requires occurrence and distribution data for contaminants of interest and the development of three-dimensional mathematical models of groundwater flow and reactive chemical transport over temporal and spatial scales relevant to water-supply planning and aquifer development. The LINJ study team has expanded the scope of a NAWQA flow-path survey to incorporate modeling and other methods to further the understanding of current and future distributions of contaminants in a vulnerable water-supply relevant aquifer-- the Kirkwood-Cohansey aquifer in the Glassboro region.

The 72 well monitoring network installed in the Glassboro region during FY96-97 was designed to randomly sample recently recharged groundwater. The network constitutes both an urban land-use survey and our initial efforts at a regionally focused flow-path survey by providing detailed information on the occurrence and distribution of specific contaminants (loadings) at the water table from all major land-use settings in the study area. This survey, when linked with land use and other data, provides the basis for spatially estimating loadings of specific compounds to the water table as the starting point (input) to the groundwater flow and transport model.

Three-dimensional groundwater-flow (MODFLOW) and particle-tracking (MODPATH) models of the surficial Kirkwood-Cohansey aquifer in the study area was also developed in FY96. The model (1) quantifies the rate and path of groundwater movement through the aquifer system under current and projected water-use scenarios, (2) quantifies the time required for water to move along given flow paths from the water table to eventual discharge at a stream or water-supply well, and (3) establishes a basis for predicting the rate of contaminant movement along various flow paths.

This flowpath analysis was used in FY97 to evaluate potential well locations for sampling groundwater at greater depth (different ages and urban-land use sources) as part of the flow path study. Based on this evaluation, 30 flow-path wells were located (along flowpaths with urban land use in their source area of recharge) and installed (except for depth, using the same protocols as the LUS) during FY97.

Proposed FPS work in FY98

Sampling of these 30 FPS wells began in October 1997 (FY98) and should be completed by December 1997. From an analysis of water-quality data from the 72 well LUS, a more targeted analytical schedules will be used in this deeper network and will include field parameters, major ions (SH2750), nutrients (SH2752), pesticides (SH2001), volatile organic compounds excluding non-target compounds (SH2021), and CFCs. Quality assurance samples will be 15% of the samples.

Chemical transport modeling to determine the relevance of compound occurrence and importance to water supply in this surficial aquifer system is proposed for FY98 and beyond. Modeling efforts will likely include an analysis of transport and fate of consevative constituents, for example nitrate, and non-conservatives, for example atrazine and/or MTBE. We will attempt to explain and possibly predict the effects of past and present land uses on the water quality of water-supply wells and streams of the study area. Our modeling analysis will heavily rely on the results of the FY97 extended LUS data (72 wells) to characterize the water-quality loadings from various land uses. The FY98 FPS (30 wells) will be used to characterize the water quality of 10-25year old water and provide data to define the movement of these constituents in this system.

Nitrate from agriculture and urban sources is known to be moving conservatively through this aerobic aquifer system. This would be our first constituent to model. Although it may be difficult to project actual concentrations discharged to streams because of the inability to define areas of denitrification along the streams, we could predict conservatively the nitrate the stream should be recieving and compare that to observed nitrate data in study area streams. We also would like to model the expected nitrate concentrations in water-supply wells given past and present land uses in the study area. Assuming conservative behavior of nitrate should be appropriate for wells as compared to streams. As a basis for water-supply well analyses, we will use nitrate/land-use relations and current land-use data to estimate current nitrate loadings to the water table. An iterative flow path-backward/path- forward model analysis with deeper/historical well data and historical land-use data would be used to refine our estimates of the histroical nitrate loadings, both spatially and temporally.

For a non-conservative analysis, at a minimum we can use a similar approach as nitrate, but with some reasonable ranges of decay/retardation and see how model results compare to our deeper well survey and water-supply well data. In this way, for example, we might be able to predict the fate of MTBE in our highly urban system. This is highly conceptual, but it is the kind of analyses that the water managers in our study area have indicated they would like to see made available.