National Water-Quality Assessment (NAWQA) Program

Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz

Land-Use Studies

The primary objective of the Land-Use Studies is to assess the concentrations and distribution of water-quality constituents in recently recharged ground water (generally less than 10-years old) associated with the most significant settings of land use and hydrogeologic conditions in each Study Unit. A closely related second objective is to understand the human and natural factors in each setting that affect ground-water quality. This focus on recently recharged shallow ground water in priority settings enables direct assessment of relations between land-use activities and ground-water quality. The potential significance of shallow ground-water quality conditions to underlying and adjacent ground-water resources is assessed through the Study-Unit Survey and the Flowpath Studies.

Selection of Land-Use Studies

Two to four Land-Use Studies typically are completed in each Study Unit during the first cycle of NAWQA. The process of selecting Land-Use Studies in each Study Unit begins by mapping the distribution of major land uses in relation to the aquifer subunits designated for the Study-Unit Survey. The land uses are defined and mapped as part of developing the Environmental Framework for each Study Unit. The intersection of land-use areas with subunits defines an initial set of ground-water land-use settings, which are then further subdivided, if necessary, to reflect important differences in hydrogeologic settings within major subunits. This process results in a variable number, frequently from 5 to 20 potential land-use settings for study.

The priority of potential Land-Use Studies is based on a combination of Study-Unit and National Synthesis priorities. Factors considered in assigning priorities include importance of the land-use setting to used ground water in the Study Unit, contamination potential of the targeted land use, geographic correspondence to subunits concurrently sampled as part of the Study-Unit Survey, geographic correspon-dence to surface-water studies, and National Synthesis plans for comparisons among land-use settings of regional and national importance. The top two to four land-use settings are chosen for investigation. An example of the geographic relation of Land-Use Study areas to the Study-Unit Survey is shown by the study design in figure 8 for the San Joaquin/Tulare Basins Study Unit. The large vineyard area in the southern part of the Study Unit was not included in the Land-Use Study because of a much different hydrogeologic setting.

Figure 8

(Click on image for a larger version, 103K)

Figure 8. Example of study designs for a Study- Unit Survey and Land-Use Study, San Joaquin/ Tulare Basins Study Unit.

Well-Selection Strategy

Wells selected for a Land-Use Study are randomly distributed throughout the occurrences of the land-use setting (combination of land use and hydrogeologic conditions) of interest within the Study Unit. The grid-based selection method described by Scott (1990) is used in this process. A minimum of 20 wells are sampled in each land-use setting. A minimum of 30 wells are sampled in land-use settings where the ground-water quality is expected to vary the most.

Many, if not most, land-use settings defined for study are not single, homogeneous, contiguous areas but exist in multiple distinct "islands" or are intermingled with other land uses. In the study approach, the selection of sampling wells targets all occurrences of the defined setting rather than a representative subarea. A possible exception would be if a large (greater than 75-percent) majority of the targeted setting is in a single area or a few large contiguous areas, but with complex boundaries and small independent areas scattered around. Because of the increased uncertainty about land-use influences near boundaries, these small, independent areas with complex boundaries sometimes are excluded.

Usually in the mapped areas of the targeted land-use setting, land use is not truly homogenous, and more than one land use can be near a well. In evaluating the effects of land use on ground-water quality, the most reliable approach is to select wells located in recharge areas, screened near the water table, and directly downgradient from the specific targeted land-use setting. This approach helps avoid the influence of other land-use activities and complications from upward movement of water that originated in distant areas. Because land use can change with time, sampling locations are selected where it has been stable over the past decade.

These restrictions lead to very selective choices of wells to sample. The wells sampled for the Land-Use Studies have short (ideally less than 3 m in length) open intervals located near the top of unconfined aquifers. Only wells located in recharge areas under-lying or immediately downgradient from the land use of interest are selected. Ideally, they are observation wells or low-capacity existing wells to avoid the complexities of determining contributing areas to heavily pumped wells. Many wells are installed by NAWQA to meet these criteria.

Sample Analyses

In general, the same national target constituents are analyzed in samples from the Land-Use Studies as for the Study-Unit Survey. The addition of other constituents varies among Land-Use Studies as in the Study-Unit Survey. Volatile organic compounds will be analyzed in samples from many of the Land-Use Studies.

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