Groundwater-Sampling Network To Study Agrochemical Effects on

Chapter 7

Groundwater-Sampling Network To Study Agrochemical Effects on Water Quality in the Unconfined Aquifer

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Southeastern Delaware Judith M. Denver Water Resources Division, U.S. Geological Survey, Room 1201 Federal Building, 300 South New Street, Dover, DE 19901 Understanding local and regional groundwater-flow patterns was necessary to design a sampling network to study the movement and distribution of agrochemicals in the unconfined aquifer in southeastern Delaware. Clusters of wells completed at various depths were installed in the expected direction of local groundwater flow along a transect from the center of a 100-ha cultivated field toward a nearby stream. Contrary to expectations, groundwater flow in the study area is almost parallel to the stream, in the direction of regional flow. Consequently, agrochemicals from the site migrate along flow paths from source (recharge) areas to distant regional discharge areas and do not significantly influence the water quality in the stream. The sampling network was expanded upgradient and downgradient from the original site during a second phase of the study. The expanded network provided better understanding of agrochemical distribution relative to regional groundwater-flow patterns. Distribution of agrochemicals in the unconfined aquifer is affected by such factors as differences in fertilizer application rates (for com and soybean crops), recharge timing and magnitude, soil and aquifer properties, upgradient land use (present and historical), and groundwater withdrawal. The degree of agrochemical influence on water quality varies widely, both areally and with depth in the aquifer, because the interrelations among these factors are complex. A three-dimensional sampling network is needed to understand the groundwater-flow system and to interpret groundwater quality in relation to the above factors. This paper presents the design of a groundwater-sampling network at a research site used for two studies of water quality in an agricultural area This chapter not subject to U.S. copyright Published 1991 American Chemical Society

In Groundwater Residue Sampling Design; Nash, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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GROUNDWATER RESIDUE SAMPLING DESIGN

(Figure 1) (1^). The importance of understanding both local and regional groundwater-flow systems when assessing agrochemical effects is emphasized. Examples of both problems and successes with this network are given. The original groundwater-sampling network was installed in an approximately 100-ha field with an irrigation well at the center (Figure 2). A major project objective was to study groundwater withdrawal effects on the distribution and movement of agrochemicals in the aquifer. A second objective was to assess agrochemical effects on water quality downgradient from the field and in an adjacent stream. Based on results of the first study, the sampling network was expanded in a subsequent project to study the distribution of agrochemicals in relation to regional groundwater flow (Figure 3). The expanded network extends from the regional recharge area, which is upgradient from significant agricultural land use, to the regional discharge area, which is downgradient from the predominantly agricultural area. Study Area The study area is in eastern Sussex County, Delaware, which is part of the Atlantic Coastal Plain Province (Figure 1). Land uses are predominantly corn and soybean production and forest. Soils are generally well-drained sandy loams. The aquifer, which is approximately 30 m thick, consists mainly of permeable sand and gravel; it is susceptible to contamination by NO^-N and other chemical constituents associated with agricultural practices. The site is one of several local watersheds being investigated in conjunction with the U.S. Geological Survey's National Water Quality Assessment (NAWQA) project on the Delmarva Peninsula (Koterba, M.T., Shedlock, R.J., Bachman, L.J., Phillips, P.J., in this volume). Original Well Network Groundwater-flow directions were estimated using a published water-table map of the area to select sites for well installation (3). A network was designed that included five clusters of wells. Individual wells within each cluster were screened at different depths in the aquifer to monitor vertical hydraulic gradients and water quality at approximately 6 m intervals from near the water table to the base of the unconfined aquifer, about 30 m below the land surface. Three of the clusters were installed in a transect between the irrigation well and the stream in the expected direction of groundwater flow (Figure 3). Several shallow wells also were installed around the field perimeter for additional water-table control. Vertical hydraulic gradients and water quality were related to irrigation pumping and natural groundwater-flow patterns.


Agrochemical Effects on Water Quality


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Figure 2. Configuration of Original Sampling Network




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Groundwater Flow and Water Quality Initial water-table measurement indicated that groundwater flow at the site is predominantly parallel to the stream, or perpendicular to the expected direction (Figure 4). Streamflow is maintained by discharge from a local shallow groundwater system which lies above the regional system (Figure 5). The local flow system around the stream apparently does not extend beyond the adjacent wooded area. Stream water does not contain chemical concentrations that can be attribbuted to agrochemicals, but rather reflects natural background conditions. Agrochemicals in the aquifer move from source areas to distant regional discharge areas. Groundwater withdrawal for irrigation promotes movement of water into deeper parts of the unconfined aquifer. This was especially apparent in the well cluster adjacent to the irrigation well where the downward hydraulic gradient was almost 1 m during pumping. The effects of irrigation pumping decrease with distance from the well: Water level measurements from the well clusters on the field's perimeter had downward hydraulic gradients of less then 2 cm (7). Water recharging the aquifer contains dissolved ions from fertilizers and lime applied to the crops. When com is planted, nitrogen is applied to the field. Soybeans, generally grown on alternate years, require no nitrogen fertilizer. As a result, recharge to the aquifer contains different amounts of nitrate and other agrochemical constituents depending on the crop and water in the aquifer is chemically stratified. This stratification was most obvious in samples from the well cluster nearest the irrigation well where water is pulled rapidly downward by pumping. In parts of the aquifer not significantly influenced by groundwater pumping, upgradient land use is the principal control on water chemistry. The effects of groundwater pumping on water quality generally were difficult to distinguish from the effects of agrochemicals carried along regional flow paths, except immediately adjacent to the irrigation well. Expanded Monitoring Network The sampling network was expanded upgradient and downgradient of the original site to study water quality in the regional flow system (Figure 3). Six well clusters screened at different depths from near the land surface to approximately 30 m below land surface were installed along with several independent shallow wells. Groundwater flow is predominantly from west to east across the expanded study area. The network successfully defines regional groundwater flow: Vertical hydraulic gradient is directed downward at the western part of the network, indicating groundwater recharge, and up-ward at the eastern part, indicating groundwater discharge (Figure 6).



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3-69 WELL LOCATION.--Number is water-table φ altitude March 9, 1987, in m above sea level. 4 01 Number is stream-bed altitude in m above • sea level. ^WATER TABLE CONTOUR.-Shows equal + ' water-table altitude above sea level. Dashed where approximately located. Contour interval 0.5 m. φ

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Figure 4. Direction of Groundwater Flow, Original Sampling Network




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In Groundwater Residue Sampling Design; Nash, R., et al.; Washington, O.C 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Relation of Water Chemistry to Land Use Water samples from wells aligned in the direction of regional groundwater flow show a wide range of NO^-N concentrations (

Groundwater-Sampling Network To Study Agrochemical Effects on

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