316(a) Thermal Variance Study

Kaw Power Station is a three-unit, steam electric power plant built in the 1950s and 1960s. Once-through cooling is used for steam condensation. The Kansas River is the water source and the receiving stream. At the time of the study the plant operated only May through September. In the plant’s discharge permit, the Kansas Department of Health and Environment set final effluent limits for temperature that the plant did not meet. However, implementation of these limits was postponed pending completion of a study pursuant to Section 316(a) of the Clean Water Act. Section 316(a) could support issuance of a permit with a variance from the water quality standard for temperature. The 316(a) study consisted of three basic parts: collecting aquatic biota, mapping the discharge plume and using a hydrodynamic plume model to extrapolate the discharge plume to the low-flow condition important to regulators.

• Plume mapping
• Plume modeling
• Aquatic biological sampling

Five sites were established for collecting aquatic biota. One site was 20 meters upstream from the plant’s outfall and the other sites were 25, 75, 175 and 325 meters downstream. At each location, samples were collected from the near shore. The intent of this layout of sampling sites was to collect organisms from along a temperature gradient. Sampling at places with different temperatures would allow for the determination of a dose-response relationship. The biota collected included fish, drifting larval fish and macroinvertebrates, and benthic macroinvertebrates. Fish were collected with 20-by-6-foot seine. At each site, three replicate hauls were made over a 20-meter segment of shoreline. Fish were collected in April, June, August and October. Drifting organisms were collected with 500-μm mesh drift nets. Drift nets were deployed one day per month, April through September. Benthic macroinvertebrates were collected using a 15-by-15-cm Ekman dredge. At each site, three samples were collected, pooled and sieved through a 500-μm mesh screen.

The discharge plume was mapped by taking numerous surface temperature measurements and at 0.5-meter depth intervals along 10 transects across the river from upstream of the discharge to 400 meters downstream. From these data, lines of equal temperature were interpolated to visualize the plume.

The discharge plume size and shape are dependent on the river flow. Cormix was used to determine the plume’s shape at the regulatory relevant low flow condition. Cormix required information on the geometry of the receiving stream, which was determined from a bathymetric map prepared from river bottom cross-section made with boat-mounted, strip-chart recording sonar.

The only adverse impacts observed were a decline in benthic macroinvertebrate density and richness downstream of the discharge. Cormix modeling indicated the thermal plume at the regulatory low-flow condition would not be larger than that observed during the study. The minimal impacts to the biota observed are not expected to increase at the regulatory low-flow conditions. The results indicate past operations have not impacted aquatic biota beyond those allowed by water quality regulations.  As such, the study supported the removal of discharge limits for temperature from the station’s discharge permit.