2012 - Rice Lake Excess Nutrient TMDL

Executive Summary

This Total Maximum Daily Load (TMDL) study addresses nutrient impairments for Rice Lake (DNR #73-0196) located in the North Fork Crow River (NFCR) watershed, HUC-8 (# 07010204), Upper Mississippi River Basin in Stearns County, Minnesota. The goal of this TMDL is to quantify the pollutant reductions needed to meet the Minnesota water quality standards for deep lakes in the North Central Hardwood Forests (NCHF) ecoregion. The numeric water quality standards for Rice Lake are summer average values of a total phosphorus concentration less than 40 µg/L, summer average values less than 14 µg/L for chlorophyll-a, and summer average values greater than 1.4 meters for Secchi depth. Water quality in Rice Lake does not currently meet state nutrient concentration standards for deep lakes in the North Central Hardwood Forest ecoregion.

Rice Lake has a direct watershed that is approximately 10,730 acres in size. The North Fork Crow River watershed drains approximately 162,122 acres above Rice Lake and flows in and out of the lake through the southwest basin. Land use in the North Fork Crow River and Rice Lake direct watersheds is predominantly agriculture (>50%); including row crops (corn soybean rotation) and animal agriculture. Rice Lake has four major basins; three of which have an average depth greater than 10 feet (L1, L2 and L4) while one basin, L3, has an average depth of 7 feet. Rice Lake has a history of carp and curly-leaf pondweed infestation.

A nutrient budget was developed for Rice Lake along with a lake response model to set Load and Wasteload Allocations. Phosphorus sources to Rice Lake include direct watershed runoff (approximately 2%), North Fork Crow River watershed runoff (approx. 93%), internal sediment release of phosphorus (approx. 4%), with the remaining phosphorus source loads coming from atmospheric deposition (approx. 1%). The TMDL allocation for Rice Lake to meet state water quality standards necessitates a phosphorus load of 29,848 pounds per year. The TMDL allocation represents a 50% reduction from current loading to Rice Lake.

One of the primary nonpoint sources of phosphorus for Rice Lake is runoff from agricultural areas, containing both row crops and animal agriculture. Based on the Unit Area Load (UAL) model and agricultural animal counts through the watershed, one of the primary nonpoint sources of nutrients from agricultural areas is from animal manure. There are over 55,000 animal units in the North Fork Crow River watershed above Rice Lake and Rice Lake direct watershed. These animals produce over 5.3 million pounds of phosphorus per year. A large portion of the phosphorus input to Rice Lake is via land applied manure practices. Nutrient management in the Rice Lake watershed will need to focus on manure management. Sediment phosphorus release rates in the deep basins of Rice Lake were high compared to typical release rates in healthy mesotrophic lake ecosystems. So while the internal nutrient load (4%) may appear small compared to the total watershed load, sediment loading should be addressed through internal load controls.