Detailed Hydrochemical Studies as a Useful Extension of National Ground‐Water Monitoring Networks

AbstractRegional and national ground‐water monitoring networks are used to inventory and to monitor diffusive (nonpoint) sources of ground‐water contamination. The Dutch National Ground Water Quality Monitoring Network (LMG) is an example of such a network, and monitors the shallow ground water of The Netherlands at two depths (10 and 25 m below land surface) in 350 wells, giving an average density of one monitoring well per 100 km2. Once water‐quality changes have been observed in time and space, the regional network is less suited to the study of the detailed chemistry, dynamics, and scale of the observed changes, because of the low density of sampling points.Two important threats to the quality of ground water that were identified by the regional network were studied in greater detail by use of multilevel observation wells along cross sections parallel to the direction of ground‐water flow. The first detailed study evaluates the fate of nitrate and other agricultural contaminants in a sandy aquifer recharged by precipitation. The second detailed study evaluates the effects of recharge from IJssel river water in a sandy aquifer.The precipitation‐recharged section consists of two hierarchically‐related ground‐water flow systems. A regional‐scale, relatively unpolluted system, recharged within an ice‐pushed ridge by rain water, is overlain by several local‐scale, agriculturally contaminated flow systems, that extend from the water table to 25 m below land surface. Locally, denitrification by organic carbon occurs in or near the unsaturated zone. Within the regional system, ground‐water composition changes over a distance of 5–6 km from aerobic infiltrating rain water to sulphate‐reduced, calcite‐saturated rain water. In the river‐water infiltration section IJssel river recharges the aquifer within a distance of much less than 1 km from the river. The main part of the section consists of local‐scale agriculturally contaminated flow systems.The varying compositions of ground water are controlled by the hydrological flow patterns, the composition of the aquifer sediments (e.g., presence of carbonate, clay minerals, and organic matter) and the composition of the source water (river water, rain water, agriculturally polluted). These controlling factors locally lead to relatively rapid transitions and heterogeneity of ground‐water compositions. The transition zones are considerably smaller than the density of observation wells in the Dutch ground‐water monitoring network, which limits the usefulness of regional monitoring networks for identifying chemically similar hydrologic zones or for effectively evaluating physical and chemical processes that affect the water quality. Regional patterns may evolve as a result of selective placement of monitoring wells, which show a specific facet of the ground‐water quality of that region.To evaluate and optimize monitoring site information and to study geochemical processes in the ground water, it is recommended that local‐scale sections be added to the national monitoring network program.