Quality of water discharged from two agricultural watersheds in southwestern Iowa

The quality of water discharged from a 157.5‐ha diversified conservation‐farmed watershed in southwestern Iowa was within acceptable limits for potable water except for the levels of ammonia, inorganic phosphorus, and coliform. Total coliform levels exceeded the established criteria on two occasions, and fecal coliform once during the 2‐year study. Nutrient concentration was high occasionally, but the nutrient quantities discharged from the watershed were low, the effectiveness of the level‐terrace system in controlling surface runoff and erosion thus being shown. Atrazine residue in the runoff and sediment was detected in only one sample. Even though several chlorinated hydrocarbon and organophosphate compounds had been applied to the watershed, none was detected in runoff or sediment. A comparison of the data from a 33.6‐ha contour‐farmed watershed with the data from the 157.5‐ha well‐planned conservation watershed showed the benefits and necessity of using some means of controlling runoff and erosion to prevent loss of agricultural chemicals that may degrade our surface waters.