Publication Summary

Groundwater beneath four land application fields of an operating dairy in Thurston County was monitored 16 months before and 44 months after an animal waste holding pond (AWHP) was constructed and used. Prior to the construction of the pond, manure and wastewater were applied to three of the fields year-round. Application methods and loading rates varied for three of the fields over the monitoring period and remained the same for the fourth field. Application methods consisted of tanker truck spreading, stationary and traveling guns, and soil injection.

Groundwater beneath four land application fields of an operating dairy in Thurston County was monitored 16 months before and 44 months after an animal waste holding pond (AWHP) was constructed and used. Prior to the construction of the pond, manure and wastewater were applied to three of the fields year-round. Application methods and loading rates varied for three of the fields over the monitoring period and remained the same for the fourth field. Application methods consisted of tanker truck spreading, stationary and traveling guns, and soil injection.

The monitoring network, consisting of 13 monitoring wells, two water-supply wells, and three surface water stations, was sampled every two months for nitrate+nitrite-N, chloride, total dissolved solids, ammonia-N, total phosphorus, total organic carbon, and fecal coliform bacteria. At the onset of monitoring, groundwater quality beneath three of four fields showed substantial impairment. Groundwater quality beneath the fourth field was less affected. Fall soil nitrate concentrations were determined for some years, and annual nitrogen loading was estimated for each field using literature values. Trend analysis of the water quality results using the Seasonal Kendall test showed significant improvement at the 95% confidence level for portions of the aquifer underlying three of the application fields. Parameters showing improvement were nitrate+nitrite-N, chloride, and total dissolved solids. The AWHP contributed to improving groundwater quality, and potential mechanisms are identified. Other factors contributing to groundwater quality improvement are discussed in the report. Ammonia-N, fecal coliform bacteria, and total phosphorus concentrations in surface water downstream of the dairy showed significant decreases at the 95% confidence level.

Link to EIM data for User Study ID BEAVCKDA