Concentration of nitrate and other water-quality constituents in groundwater from the water table beneath forage fields receiving seasonal applications of dairy manure, Whatcom County, Washington (2015)

Application of dairy manure to cropland is intended to provide nutrients for crop growth and improve soil quality, the goal being to match the rate of nutrient application to that rate of nutrient removal by the crop. Nitrate contamination of groundwater is widespread and persistent in the shallow surficial aquifer of northwestern Whatcom County where dairy farming and forage production is a primary land-use activity. A study to test an alternate strategy for scheduling manure application to fields based on hydrologic properties of specific soils and fields, measurements of manure and soil parameters, and current and forecasted precipitation for the three days immediately following manure application is being evaluated by the Whatcom County Conservation District. To compare the effects of manure scheduling on leaching losses to groundwater concentrations of nitrogen in the form of nitrate, ammonia, and total nitrogen, along with chloride, phosphorus, and bacteria were measured in groundwater beneath dairy forage fields receiving seasonal application of dairy manure use either conventional or alternative manure application scheduling. Samples of groundwater were collected from wells screened across the water table and using an inflatable packer to isolate the upper six inches of the saturated zone from deeper portions of the well. Groundwater samples were collected from three fields at approximately monthly intervals for periods ranging from 1 to 3 years. Nitrate concentrations in groundwater from near the water table exceeded US EPA drinking water contaminant levels of 10mg/L in most samples. Concentrations of nitrate and other water quality constituents varied seasonally.