Groundwater loading of nitrate-nitrogen and phosphorus from watershed source areas to an Iowa Great Lake

Groundwater discharge to a lake can be an important component to water and nutrient budgets. In this study, we evaluated groundwater loading of nitrate-nitrogen (NO3-N) and phosphorus (P) to West Lake Okoboji, Iowa, using a watershed-based approach based on groundwater recharge and land cover class. Our objectives were to assess groundwater level fluctuations and nutrient concentrations under representative land use classes and develop an allocation model for groundwater nutrient loads based on land cover class. Monitoring wells were installed at 21 locations around the lake and sampled during a three-year study period. Groundwater quality varied among the land cover types with average NO3-N concentrations the highest beneath cropped fields (8.8 mg l− 1) and residential areas (2 mg l− 1), and P concentrations ranging between 0.05 and 0.1 mg l− 1 throughout the region. NO3-N loads were the highest under cropped fields and this source accounted for approximately 90% of the NO3-N, whereas P loads were more evenly distributed among source areas. Groundwater recharge averaged approximately 76 mm year− 1 for vegetated areas and substantially less for urban areas. Based on mass balance, groundwater discharge may account for 80% of the NO3-N in the lake compared to 10% of the P. Results are instructive to more effectively target implementation of conservation practices to major nutrient loading areas for reduction of NO3-N and P delivered to the lake.