BIG SPRING BASIN WATER-QUALITY MONITORING PROGRAM: DESIGN AND IMPLEMENTATION
J.P. Littke and G.R. Hallberg
Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 91-1, 1991, 19 p.
The agricultural practices, hydrology, and water quality of the Big Spring basin have been studied since 1981. A network of precipitation stations, tile lines, streams, springs, and wells of various depths has been monitored during this period. These investigations documented changes in water quality related to historic changes in cropping practices, nutrient management, and fertilizer and chemical use. Based on this research a multi-agency group initiated the Big Spring Basin Demonstration Project in 1986 to integrate public education with on-farm research and demonstration projects that stress and monitor the environmental and economic benefits of prudent chemical management. The water-quality monitoring network was expanded to over 50 sites, to provide a detailed record of the water-quality changes accompanying improved farm management. The monitoring network is designed in a nested fashion, from small-scale field plots to the basin groundwater and surface-water outlets. Ten key sites have been instrumented for continuous or event-related measurement of water discharge and chemistry and for automated sample collection. Four sites have been instrumented with deep-nested monitoring wells, penetrating the uppermost bedrock aquifer. The development of monitoring sites within the Big Spring basin has been a cooperative effort among the Iowa Department of Natural Resources, the US Geological Survey, Iowa State University, the USDA-Soil Conservation Service, and the US Environmental Protection Agency.
The network design and instrumentation allows a detailed view of the hydrologic system, at a variety of scales. The smallest areas with instrumented tile lines and/or shallow piezometers are individual fields or land-use tracts (5 to 40 acres) with known management. Nested within some of the individual fields are research and demonstration plots (<1/4 acre) of varied management, and within selected research plots, microplots (3 sq. ft.) are used. Monitoring at the field scale allows observation and interpretation of the processes of water and chemical transport in relation to soil properties and agricultural management. Water quality improvements caused by changes in agricultural practices will most quickly and clearly become apparent at the field scale.
From the individual field sites, the nested monitoring scheme follows the natural hierarchy of the drainage system. Watersheds of increasing size are instrumented and monitored, up to the main surface-water and groundwater outlets for the basin (103 sq. mi.). Water quality at these larger scales is an integration of the management practices of all the individual parcels of land they contain. Water quality improvements at these increasingly larger scales will require longer periods of time to become apparent, relative to field plots.
The hydrologic and chemical responses of the individual fields to recharge events can be tracked through the larger groundwater and surface-water systems. While the concentration changes are not as great or as immediate at the largest scales monitored, they are clearly apparent and the nested monitoring design employed allows the pulse to be interpreted in relation to their source. Through this hierarchy responses to changes in management practices can also be tracked at various scales, and a detailed record of the chemical flux through the basin is being established. This will afford, over time, an assessment of the water-quality improvements resulting from changes in farm management.