Masters Defense

Assessing Hydrologic and Land-Use Controls on Water Quality in Wild Rice (Manoomin/Psiŋ) Lakes in the Otter Tail River Watershed, MN

Manoomin (Ojibwe language), or Psiŋ (Dakota language)/Wild Rice/Zizania palustris, is an ecologically important aquatic plant that holds deep cultural importance across the Upper Great Lakes, especially for Indigenous peoples in the region. It has thus been of considerable concern that it has been declining since Euro-American settlement. Indigenous harvesters and local knowledge holders attribute its decline in recent decades in part to the expansion of intensive agriculture – an interaction that has not been previously researched. As part of a tribal-university collaborative centered around prioritizing Indigenous perspectives, this work was conducted in partnership with the White Earth Nation to examine how different environmental variables together drive excess nutrient dynamics in the Otter Tail River Watershed (OTRW), where a sharp decline in Manoomin has been observed to coincide with a major shift to commercial agriculture since the 1970s. Field hydrological monitoring over two growing seasons at three representative Manoomin lakes in the OTRW suggest that water quality in the root zone of Manoomin lakes may be threatened by both surface runoff and groundwater inputs of agricultural pollutants, including nutrients. Four regression random forest models were conducted on surface water and groundwater nitrate and total phosphorus datasets compiled across the OTRW to determine the relative influence of agricultural land use, soil texture, and climate on nutrient concentrations, and to identify the various fate and transport pathways spanning the watershed. Processes at the land surface, such as short-term (multi-day) climate, emerged as most important for predicting surface water quality overall, with the influence of soil interactions varying by nutrient type. In particular, high surface water nitrate was more strongly related to the presence of high infiltration soils and greater short-term precipitation compared todense agricultural cover, indicating that nitrate may be commonly transported through groundwater pathways to surface water bodies. In contrast, higher surface water phosphorus was predicted to align more with high clay content and agricultural cover than climate variables, suggesting that erosive surface runoff may be the main transport mechanism from agricultural fields. A threat to both drinking water supply and surface water quality, shallow and deep groundwater nitrate exhibited strong horizontal spatial correlations and were found to be most dictated by high-infiltration soils and long-term (5- and 10-year) climate trends. These results reveal potential multi-year lag times that reflect the accumulation and persistence of pollutants in groundwater, as well as their ready spread through the subsurface, underscoring the spatiotemporal complexity of groundwater contamination. This research contributes to a broader understanding of water quality and possible hydrological pathways of agricultural pollutants in the OTRW, and it sheds light on the risks to ecologically and culturally important Manoomin lakes throughout the watershed.