Reservoir and Lake Nutrient Criteria A Different Approach D.V. Obrecht, J.R. Jones & M.K. Knowlton – MU Limnology

UMBRELLA APPROACH 1) Reference reservoirs and lakes – 75 th percentile 2) All reservoirs and lakes – 25 th percentile 3) EPA’s 304(a) criteria

Oxbow lakes (n=12) TP = 212 ug/L TN = 1.56 mg/L Reservoirs (n=135) TP = 45 ug/L TN = 0.73 mg/L

Reservoir TP (µg/L) range of geomeans Maysville (n=10) – 300 Grindstone (n=5) Unionville (n=10) Long Branch (n=20) Viking (n=16) Forest (n=19)

STEP APPROACH Designated Use Impairment of use Algal biomass Nutrient levels Criteria level

Drinking Water Supply Impairments -taste and odor -clogging of filters -algal toxins

There may be too many factors that influence water quality and too much variability within and among systems to allow for the setting of a single set of criteria to be used by the state for regulation.

EPA allows some flexibility: …(states can) develop their own criteria which reflect more locally representative conditions. …prioritize their waters…..Such an approach should include a mechanism for evaluating the sensitivity of all waters…considering current and expected land use… EPA memorandum, Nov. 14, 2001

A Different Approach!

A lake is a reflection of its watershed.

Cropland (%) Total Phosphorus (µg/L)

A reservoir is also a reflection of its watershed, and the intensity of that reflection is dictated by hydrology.

Regression model results Equation r 2 TP = %crop0.62 TP = %crop – 0.50DH0.73 TP = crop% DH FI0.77 DH is dam height, a surrogate from reservoir morphology FI is flushing index

Proportion crop Short Res. Time Med. Res. Time Long Res. Time > 40% 20 – 40% 10 – 20% 1 – 10% < 1%

Proportion crop Short Res. Time Med. Res. Time Long Res. Time > 40%116 ug/L75 ug/L47ug/L 20 – 40%97 ug/L80 ug/L53 ug/L 10 – 20%59 ug/L54 ug/L33 ug/L 1 – 10%40 ug/L27 ug/L < 1%17 ug/L

Can we use agriculture to classify reservoirs? USGS photo

Reservoirs were built into landscapes that had already been altered. No Restoration Water quality in a reservoir is a function of morphology/hydrology and location within the landscape.

7,600 – 34,000 ac 34,000 – 58,000 ac 58,000 – 87,000 ac 87,000 – 169,000 ac Harvested acres of corn, counties 26 counties 36 counties 22 counties Between 11% and 21% of total Missouri land surface dedicated to just corn production in 1920!

Missouri’s reservoirs >10 acres in size Year completed # %

This approach allows the state to: -Identify and protect the reservoirs that have low watershed impacts. -Identify and focus efforts on the reservoirs that have higher nutrient concentration than expected, given watershed land use and hydrology. -Gauge the potential for successful nutrient reduction by looking at the factors that control in-reservoir nutrient concentrations. And focus limited resources ($$) on those reservoirs where improvements can be made.