GROUND WATER NUTRIENT LOADING TO SURFACE WATERS, HELENA VALLEY, MONTANA

The Helena Valley is located along the eastern margin of the Rocky Mountains of west-central Montana, with a watershed situated between the continental divide to the west, and the Missouri River to the east. Ground water in the downgradient area of the valley shows a strong upward vertical gradient which generally increases with depth. Lake Helena, created by Hauser Dam on the Missouri River, fills the lower valley elevations with lake levels controlled by the Missouri River. The creation of Lake Helena raised the water table in the lower valley, mitigated by installation of a series of tile drains to lower the water table for agriculture. The drains route ground water back to Lake Helena through a series of surface water ditches. The Helena Valley Non-Point Source Assessment Project utilized drains as sampling points for ground water to characterize and differentiate nutrient loading to the system from agriculture and areas with high densities of septic systems. The sampling program included Nitrate and total Phosphorus at 12 locations on an approximate bi-weekly frequency between Spring 2013 and Fall 2014. Specialized sampling included isotopes of Oxygen and Deuterium of water for several events, and Nitrogen and Oxygen isotopes of dissolved nitrate. Nutrient loading rates are estimated using flow measurements from drains.

The detailed data sets from 12 locations show seasonal variations to loading rates across the valley, with increasing nitrogen and phosphorus during summer irrigation season and declines during the winter. The nutrient data coupled with chloride/bromide ratios show variable loading rates of nutrients to groundwater, suggesting a relationship with agriculture. In other areas, irrigation waters appear to flush nutrients derived from septic systems from the top of the water table. The isotope data support conclusions based on upgradient land use patterns to partially distinguish between agriculture and septic systems as nutrient sources to ground water. The project results are used to develop recommendations for BMP applications as a component of a Watershed Restoration Plan for the Lake Helena Watershed.