Big Spring Basin Demonstration Project
Publications on this topic.
The agricultural practices, hydrology, and water quality within the Big Spring basin, a 103 mi2 (267 km2) groundwater basin in Clayton County, Iowa, have been extensively studied by the Iowa Department of Natural Resources, Geological Survey Bureau since 1981 (Figure 1). Historic water quality data showed regional increases in nitrate in the groundwater of the basin paralleling increases in nitrogen fertilizer use and increasing corn acreage from the 1960s to the 1980s (Figure 2). The Geological Survey Bureau became involved in response to the local population's concerns about increasing nitrate concentrations in their water supplies. A network of sites, including tile lines, streams, springs, and wells, was established to monitor water quality.
The Galena aquifer (Ordovician), the main groundwater source in the basin, discharges through Big Spring at a state-owned fish hatchery. The Galena aquifer is a carbonate aquifer with moderate karst development. The extent of the groundwater basin was defined by mapping the potentiometric surface of the Galena aquifer, by dye tracing via sinkholes, and by the gaging of gaining and losing stream reaches. A water distribution system at the hatchery allows the spring's discharge to be monitored and stream gaging stations within the basin allow monitoring of surface-water discharge. Over 85% of the groundwater discharged from the basin flows through Big Spring. Surface water is discharged by various streams, but dominantly by Roberts Creek. The drainage area of Roberts Creek accounts for 65% of the basin's surface area and 75-80% of the surface-water flow leaving the basin. Normally, 90% of Big Spring's recharge is from diffuse flow through the soil and 10% is from sinkhole run-in.
Landuse in the basin is almost entirely agricultural. There are no significant point sources that impact groundwater quality. These conditions allow unambiguous study of the agricultural ecosystem. By monitoring water quality and discharge of surface water and groundwater in the basin, the mass flux of nutrients and chemicals applied within the basin can be quantified, allowing assessment of chemical balances on a basin-wide scale.
A network of precipitation stations, tile lines, streams, springs, and wells of various depths has been monitored since 1981. Early water-quality 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 (BSBDP) 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 was 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 U.S. Geological Survey, Iowa State University, the USDA-Natural Resources Conservation Service, and the U.S. Environmental Protection Agency.
Demonstration Project Success
Progress reviews in the Big Spring basin show that area farmers have reduced their nitrogen fertilizer use by 34% since 1981. The average fertilizer-N rate on all corn rotations was 115 lbs/ac in 1993 compared to 174 lbs/ac in 1981 (Table 1). In 1993 the project reduced nitrogen use and loading in the basin by nearly 2,000,000 pounds compared to 1981 practices. This is an average reduction of five tons each for the 200 basin farmers. Yields have not been sacrificed. The basin average for continuous corn from 1981 to 1993 was higher than the Clayton County average. Yields for corn after alfalfa were even higher. Frequent rains in 1993 lowered yields in the basin and across Iowa, but in 1992 the basin's corn yield average reached a record high (165 bu/ac). While corn yield variations have been primarily related to the weather, it is clear to area producers that they can reduce their impacts on the environment and do so profitably. These nitrogen fertilizer reductions have meant a cost savings of $360,000 in comparing 1993 with 1981. This is an average savings of $1,800 for each producer. Such significant reductions are the most clear form of pollution prevention and are expected to result in improved water quality during the coming years. Such inputs as nitrogen are very energy consumptive and the reductions in 1993 can be equated to a savings of over 500,000 gallons of diesel fuel, or 2,500 gallons per farmer.
The BSBDP is part of Iowa's Agricultural-Energy Initiative, a program designed to promote integrated farm management practices that improve both the profitability and the environmental performance of crop production in Iowa. The Agricultural-Energy Initiative has had a major impact not only in the Big Spring basin, but statewide as well. Statewide nitrogen loading has been reduced by over 1.2 billion pounds, with no decline in crop yields. This translates into a savings of over $180 million dollars to Iowa farmers! Within the Big Spring basin, purchased nitrogen application rates on continuous corn decreased by seven pounds per acre and purchased nitrogen use on corn following alfalfa decreased by four pounds per acre from 1991 to 1993. Based on the 1994 Big Spring Crop Production Survey, 41% of the respondents reduced herbicide rates, and 34% reduced insecticide rates over a three-year period. Statewide, pesticide reductions have also been accomplished, but they are difficult to quantify.
Water Quality Results
The Big Spring groundwater system receives both infiltration and runoff recharge, which have unique chemical signatures. These signatures can be tracked through the nested monitoring network from the soil zone beneath individual fields to the basins surface- and groundwater outlets. Infiltration recharge is enriched in nitrate and other chemicals that are mobile in soil, while runoff recharge has lower concentrations of such compounds, but is enriched in herbicides and other chemicals with low soil mobility. Typically Big Spring yields groundwater delivered through infiltration, but following significant precipitation or snowmelt, sinkholes within the basin may direct surface runoff into the aquifer, mixing it with the groundwater. As this runoff recharge moves through the groundwater system and discharges from Big Spring, relatively low nitrate and high herbicide concentrations occur during peak discharge periods (Figure3). This is typically followed by higher nitrate and lower herbicide concentrations as the associated infiltration recharge moves through the hydrologic system. During prolonged recession periods, nitrate and herbicide concentrations slowly decline as an increasing percentage of the discharge is relatively older groundwater from the less transmissive parts of the flow system.
Analysis of annual data for water years (WYs; October 1 through September 30) 1982 through 1998 shows that while flow-weighted (fw) mean nitrate concentrations and loads tend to parallel the volume of water moving through the soil and into the groundwater system, fw mean atrazine concentrations and loads do not (Figure 4). Relatively high concentrations and loads of atrazine have occurred during some years with low groundwater discharge and relatively low concentrations and loads have occurred during years with high groundwater discharge. Analysis of water-quality changes is complicated by climatic variations, subsequent storage effects, and system time-lags. Therefore, before declines in nitrate-N and atrazine loads can be attributed to improved management and source reduction, overall system variations must be considered.
Relating the decreases in nitrogen fertilizer use in the basin to changes in nitrate concentrations in Big Spring groundwater has been problematic. The gradual reductions in nitrogen fertilizer and herbicide use, such as atrazine, resulting from improved management practices have largely been obscured by year-to-year variations resulting from climatic variability, particularly the variability of rainfall. The driest consecutive two-year period in Iowa's history, WYs 1988 and 1989, preceded the wettest consecutive two-year period, WYs 1990 and 1991. From WY 1988 to WY 1989 annual discharge declined from 26,008 to 12,672 acre-feet (ac-ft), annual fw mean nitrate-N concentration decreased from 9.5 to 5.7 mg/L, but annual fw mean atrazine concentration increased from 0.13 to 0.61 µg/L. From WY 1990 to WY 1991 annual discharge increased from 17,476 to 42,481 ac-ft, annual fw mean nitrate-N concentration increased from 8.2 to 12.5 mg/L, and annual fw mean atrazine concentration increased from 1.06 to 1.17 µg/L. From WY 1992 to WY 1993 annual discharge increased from 37,278 to 58,186 ac-ft, annual fw mean nitrate-N concentration decreased from 12.0 to 11.4 mg/L, and annual fw mean atrazine concentration increased from 0.22 to 0.27 µg/L. In WY 1994 discharge decreased to 31,266 ac-ft, fw mean nitrate-N concentration decreased to 10.4 mg/L, and fw mean atrazine concentration decreased to 0.21 µg/L. During WY 1995 annual discharge decreased to 30,013 ac-ft, fw mean nitrate-N concentration decreased to 10.1 mg/L, and fw mean atrazine concentration decreased to 0.12 µg/L. During WYs 1996 and 1997, discharge continued to decline from 28,143 to 22,943 ac-ft, as annual precipitation increased from 30.59 to 38.29 inches. The fw mean nitrate-N concentration decreased from 10.3 to 9.7 mg/L, and the fw mean atrazine concentration decreased from 0.27 to 0.17 µg/L. In WY 1998 annual precipitation increased to 41.21 inches, annual groundwater discharge increased to 35,713 ac-ft, the fw mean nitrate-N concentration increased to 12.5 mg/L, and the fw mean atrazine concentration decreased to 0.12 µg/L.
During the WYs 1982-1998 period, the greatest annual fw mean nitrate-N and atrazine concentrations and atrazine load occurred in WY 1991. The greatest nitrate-N load was discharged in WY 1993. The smallest annual fw mean nitrate-N concentration and load occurred in WY 1989. The smallest annual fw mean atrazine concentration occurred in WYs 1995 and 1998, and the smallest annual atrazine load was discharged during WY 1988.
The increases in annual atrazine concentrations and loads following WY 1988, and the decreases in concentrations and loads from WYs 1990 and 1991 to WYs 1992 and 1993 are probably related to changes in the timing and intensity of rainfall and in the relative proportion of infiltration versus runoff recharge composing Big Spring's discharge. The precipitation events during WYs 1990 and 1991 were less evenly distributed than events during WYs 1992 and 1993. Atrazine concentrations were relatively high during January through March, and June and July of WY 1990, and in May and June of WY 1991. These high atrazine concentrations, combined with numerous runoff events, contributed to increases in annual atrazine concentrations and loads. During WYs 1992 and 1993, the short-lived increases in atrazine concentrations that occurred were usually not coincident with runoff events and as a result, the annual fw means and loads for atrazine remained relatively low.
Retardation of atrazine transport to and through the groundwater system, and annual changes in the mass of atrazine present on the land surface, are also important factors affecting changes in atrazine concentrations and loads. Pesticide degradation rates vary with environmental factors, such as soil moisture. The low soil moisture conditions during WYs 1988 and 1989 may have inhibited hydrolysis and microbial activity, which are important degradation processes. The dry conditions may also have left a greater than normal mass of herbicide available for mobilization and transport during WYs 1990 and 1991. The higher soil moisture conditions during WYs 1990 and 1991 may have enhanced hydrolysis and microbial activity, and left a smaller than normal mass of herbicide available for mobilization and transport to groundwater during WYs 1992 and 1993.
Water Year 1993 was the first year of monitoring that the annual fw mean nitrate-N concentration decreased while the annual groundwater discharge increased, and WY 1996 was the first year that the annual fw mean nitrate-N concentration increased as annual discharge decreased. These unusual changes may be the result of several factors.
A number of large runoff events occurred during the latter half of WY 1993. Nitrate concentrations typically decrease during runoff events and increase as the discharge is receding. If a greater than normal proportion of annual discharge is composed of runoff, this would lower the annual fw mean nitrate-N concentration, while if a greater than normal proportion of annual discharge is composed of infiltration recharge, this would increase the annual fw mean nitrate-N concentration. The greater than normal precipitation during WY 1991, and the significant infiltration recharge during the first half of WY 1992, would have led to increased leaching, leaving a smaller than normal mass of nitrate available for transport during WY 1993. The small number of runoff events during WY 1996 would have led to an increased proportion of infiltration recharge composing the annual discharge, leading to an increase in the annual fw mean nitrate-N concentration. The decline in annual discharge as annual precipitation increased during WYs 1996 and 1997 suggests that a large percentage of precipitation likely replenished soil moisture and infiltrated the less transmissive portions of the groundwater system that had been depleted during WYs 1994 and 1995.
It is possible that the gradual reductions in nitrogen fertilizer applied within the basin are beginning to affect changes in the water quality of Big Spring. While some of the decline in nitrate concentrations and loads during the last few years of monitoring may reflect a decrease in nitrogen applications, some of the decrease is caused by the decline in water-flux through the basin's hydrologic system. Comparisons of annual changes in corn acreage and nitrogen and atrazine applied within the Bugenhagen and Silver Creek sub-basins with annual fw mean nitrate and atrazine concentrations and loads from the tile lines draining the sub-basins show that time lags and climatic effects can overshadow any clear relationships between landuse changes and water-quality responses even at relatively small watershed scales.
During 1983, the Payment-in-Kind (PIK) program provided the opportunity to evaluate the results of a one-year reduction in nitrogen applications of about 40% in the basin, or almost double that which has accrued year-by-year over the last decade (Figure 2). Statistical analysis of the relationship between discharge and nitrate concentrations at Big Spring suggests the significant decline in concentrations during 1985 was related to the major reduction in nitrogen inputs during 1983. The incremental reductions in nitrogen fertilizer and pesticide use in the basin may not result in pronounced water-quality changes, but they will be detectable over time. Within large-scale watersheds such as Big Spring, many landuse and management practices are integrated, and water-quality responses are dampened and complicated by climatic variations, storage effects, and biochemical processing in both surface- and groundwater systems. Policy makers and planners must be aware of the time lag involved at these larger watershed scales and make appropriate commitments to long-term support.
The Big Spring Basin Demonstration Project of the Iowa Department of Natural Resources has been supported, in part, through the Iowa Groundwater Protection Act and Petroleum Violation Escrow accounts, and other sponsoring agencies: the U.S. Environmental Protection Agency, Region VII, Kansas City, Nonpoint Source Programs; the USDA- Natural Resources Conservation Service; the Iowa State University Extension Service; the University of Iowa Hygienic Laboratory; and the Iowa Department of Agriculture and Land Stewardship, Division of Soil Conservation. Full project funding through state oil-overcharge funds for the BSBDP ended during 1992. Monitoring efforts and farm management implementation projects have been scaled back and refocused, but will continue through the Northeast Iowa River Basin Demonstration Project (NEIDP). This project is funded under the USDA Water Quality Incentive Project, administered by Iowa State University Extension Service. Some funding for demonstration and education programs within the basin has been secured through the NEIDP, but further funding is needed, particularly for water-quality monitoring.