Sampling and analysis of selected water quality constituents in agricultural runoff

Water pollution from point and nonpoint sources has resulted in the passage of two major laws to enforce high quality in effluents leaving these sources. Runoff leaving agricultural land could carry with it readily soluble materials, sediment, nutrients, and other chemicals. When land has been sewage sludge-amended, heavy metals could also be carried away in runoff waters. With sediment as the greatest water pollutant on a volume basis and with recent interest in applying sewage sludges to agricultural lands for renovation, a fuller understanding is needed of the fate of sediment and metals which leave agricultural lands.

This study was divided into two sections; sediment, and metals. The purposes of the sediment section were to determine the representativeness of some techniques used to sample sediment and to develop a method for predicting sediment yield. The purpose of the metals section was to determine metal discharge in the runoff from land amended with municipal sewage sludges. The study was conducted on two watersheds at the Middle Tennessee Agricultural Experiment Station near Spring Hill and two watersheds at the Milan Experiment Station. One watershed was sewage sludge—amended. Runoff samples were collected from each watershed by an automatic pumping sampler and analyzed for sediment and selected metal concentrations.

Examination of the multiple determinations of sediment concentration in the runoff samples was done to establish justification for replacement with single determinations. Sampling of the entire stream cross-section was examined to establish validity of one point sampling. The runoff rate-weighted method for computation of sediment yield was done to establish justification for replacement with the arithmetic mean method. Data from generating watersheds were used to develop equations to predict sediment yield. Regional, local, and seasonal variations of sediment yield were investigated.

Results indicated that multiple determinations for sediment concentration of the runoff samples must be done. Network sampling of the stream should be employed. The runoff rate-weighted method needs to be utilized to calculate accurately sediment yield. Regional and local variations of sediment yield were significant with watershed differences accounting for these variations. Seasonal variations of sediment yield were significant with differences of cropping and tillage practices and rainfall characteristics accounting for these variations.

Metal concentrations in the sewage sludge were evaluated according to recommended standards. Metal discharges in runoff from one watershed were evaluated within runoff events, for runoff events during each year, and for runoff events throughout the study period. Occurrence of metals in groundwater, corn leaf, and soil from this watershed was investigated.

Concentrations of cadmium and nickel in the sludge frequently exceeded the recommended sludge standards. Cadmium and copper could pose potential threats to runoff and groundwater. There was no increased availability of metals in either corn leaf or the soil in time.