Long-term research projects in Iowa have examined a number of factors related to the transport of sediment and agricultural chemicals to surface and ground water resources. Studies conducted in Iowa have shown a similar relationship between agricultural drainage and water quality found in other areas of the Midwest. A combination of water soluble agricultural chemicals and large amounts of water moving from the soil volume into the drainage system can have significant water quality impacts.
In a five-year monitoring study (1976-1980) in east-central Iowa, measurements of nutrients and herbicide concentrations and losses were made in runoff and drainage water from cropland and in a stream draining an agricultural watershed. Herbicide concentration and losses (alachlor, metribuzin, propachlor, cyanazine, and atrazine) were greatest after spring application with in-stream concentrations reaching about 100 micro-g/L for individual herbicides in individual samples during peak runoff. This concentration exceeds or matches the drinking water standards for these chemicals. With the exception of atrazine, herbicide concentrations between peak runoff events, and over the winter were generally below detection (0.1 micro-g/L). During those periods, atrazine concentrations often ranged between 0.1 and 2.0 micro-g/L. The drinking water standard for atrazine is 3.0 micro-g/L. Annual losses of herbicide applied in the watershed were less than or equal to 2%. In-stream nitrate-N concentrations averaged about 9 mg/L, but exceeded the 10 mg/L drinking water standard much of the time when stream flow was dominated by leaching water in the form of base flow and artificial subsurface drainage (for which nitrate-N averaged 12 mg/L).
To reduce agricultural chemical loss to water resources, research has focused on chemical movement through the root zone into subsurface drainage systems. One particular area of interest has been preferential flow paths. If the surface soil becomes saturated, as it does when runoff begins, some water may move through preferential flow paths, or macropores, and leach deeper or more quickly than would be expected if the water had to flow through the soil matrix. Rainfall simulations and analysis of preferential and matrix flow conducted in Iowa showed that preferential flow facilitated the movement of a water soluble tracer (potassium bromide) and nitrate-N. However, if a chemical adheres well to soil, macro-pores can be beneficial, or used to advantage, because they cause water to bypass the soil volume where the chemical resides.
Research in Iowa has also focused on the use of agricultural drainage wells as outlets for subsurface drainage systems. In north-central Iowa, many closed depressional areas, or "potholes" are drained with surface outlets to subsurface drainage systems. However, these surface intakes may allow concentrated contaminants in pothole drainage water to enter a shallow aquifer. A main concern is elevated levels of nitrate-N found in drinking water wells. Proposed solutions to the problem include improved N management and the elimination of surface flow to agricultural drainage wells.
The Walnut Creek watershed is typical of relatively young geologic development characterized by low relief and depressional areas called potholes. This watershed is the site of a long-term study to evaluate the impact of farming practices on water quality at the watershed scale. The objectives of the study are to: (1) evaluate the loadings of nitrate-N, atrazine, and metolachlor from subsurface drained fields, subwater-sheds, and the entire Walnut Creek watershed; and (2) to evaluate the temporal patterns of drainage relative to precipitation and crop water use patterns. Farming practices in Walnut Creek are typical of the Midwest region in terms of loadings onto the soil surface, and agricultural chemical movement through subsurface drains.
Surface water samples are collected weekly throughout the year as part
of the Walnut Creek Watershed Project. Monitoring efforts were
designed to evaluate the effect of individual field practices,
aggregated effects over subwater- sheds, and agrichemical loadings from
the entire watershed on surface water quality.
Results from 1992-1995 indicate stream discharge closely mimics subsurface drainage flow patterns. Surface runoff events contribute less than 1 percent of the total discharge from Walnut Creek. This evaluation shows that subsurface drainage is the primary flow path for soluble agricultural chemicals from Walnut Creek. Relative to stream discharge, cumulative loads for nitrate-N reflect the same pattern at the three scales measured: field, subwatershed, and watershed scale (see figure). The highest nitrate-N concentrations observed were at the smaller scales; however, the relative changes among the three locations were not different. Nitrate-N concentrations in drainage water vary slightly from field to subwatershed to watershed scales. The magnitude of the loads is dependent upon the discharge volumes.
Results show nitrate-N concentrations are largest in the spring when evaporation rates are low and the greatest discharge rates occur. In watersheds that have artificial subsurface drainage, reducing nutrient loads requires more than soil testing as a Best Management Practice. Reducing loads also requires innovative methods of drainage water management, possibly altering the seasonal pattern of drainage discharges and associated nitrate-N, or reducing chemical concentrations at subsurface drains.
Cumulative distributions of nitrate-N loads from the field (22 ac) and
subwatershed (904 ac) scales, and the entire Walnut Creek watershed
(12,676 ac) for the period from 1992 through 1995 (1 kg equals 2.2 lbs).