Determination of Waconda Lake Releases Needed for Diluting Saline Water in the Solomon River at Beloit for Municipal Water Supply
by Donald O. Whittemore
A Report Prepared for the
Kansas Department of Agriculture, Division of Water Resources
and Kansas Department of Health and Environment
Kansas Geological Survey
Open-File Report 2003-49
Water supply diversion from the Solomon River at Beloit. Photo by Mark Billinger.
The City of Beloit diverts water from the Solomon River for its municipal supply. During very low flows, the river water can sometimes exceed the secondary drinking water limits of 250 mg/L for both chloride and sulfate concentrations. The City has a right for accumulating water in Waconda Lake that could be released for water supply and to reduce the salinity of the Solomon River water at Beloit. The State of Kansas needs to know the minimum amount of flow that can be released from the lake to achieve usable water quality while maximizing conservation of lake water, especially during periods of low inflows to the lake. The Kansas Geological Survey (KGS) conducted a study to determine the sources, variability, and flow relationships of chloride and sulfate in Solomon River waters entering and released from Waconda Lake and used by Beloit for public water supply, and applied these relationships to determining the lake releases needed for diluting saline water in the river at Beloit to meet the secondary chloride limit.
Historic U.S. Geological Survey (USGS) and Kansas Department of Health and Environment data indicate that the conditions are rare during which the chloride concentration of the Solomon River at Beloit exceeds the secondary drinking water limit of 250 mg/L. The flow during these conditions was <10 ft3/sec. The perception of many days when the river water apparently exceeded a 250 mg/L chloride value was based on City of Beloit measurements that appear to have analytical errors. Comparison of chloride measurements made by the KGS for this study with City analyses made on the same day indicate that the City values overestimated chloride concentration by an average of 22% during 2002-2003. The period during which samples were collected and analyzed for this study was relatively dry and represented conditions when the river water salinity would be expected to be greater than average. After adjustment of the City chloride measurements and elimination of anomalous high values that are inconsistent with the prior and subsequent concentrations, there are only two periods when the chloride content might have actually exceeded 250 mg/L during October 1988 to May 2003. The flow in the Solomon River during these two periods was generally in the 15-20 ft3/sec range at the USGS gaging station below Glen Elder Dam.
Increases in the outlet release from Waconda Lake needed to dilute chloride concentrations in the Solomon River at Beloit below 250 mg/L are expected to be small. An equation for calculating the outlet release is based on a load calculation (flow times chloride concentration)
ΔF=[1.13xFR1 (CB1-CB1)] / CB2 - CR)
ΔF is the increase in flow released from Waconda Lake, ft3/sec,
FR1 is the outlet flow from the lake before the added release, ft3/sec,
CB1 is the chloride concentration of the Solomon River at Beloit before the release, mg/L,
CB2 is the chloride concentration of the river desired at Beloit, mg/L
and CR is the chloride concentration of the lake water (outlet flow), mg/L.
The equation predicts that an increase of 2.6 ft3/sec in the outlet flow would dilute a chloride concentration of 270 mg/L in the river at Beloit to 240 mg/L, given an existing outlet release of 10 ft3/sec and a lake chloride content of 107 mg/L (the average content observed during the study). Graphs are included that show the lake releases needed for dilution based on different existing releases and chloride concentrations at the dam outlet and Beloit. The calculated flow increases for chloride dilution are within the range that could be achieved by the City of Beloit using a portion of the 2,000 acre-ft that it has the right to store in Waconda Lake.
The greatest uncertainty in the flow release equation is the change in Solomon River flow between Glen Elder Dam and Beloit. This uncertainty is the result of changes in ground-water discharge, tributary inflows, and changes in the bank storage contribution to streamflow. Each component varies with the river-water level after an increased release in addition to seasonal changes in the ground-water levels in the alluvial aquifer. Use of the average chloride concentration for the inflows from the North and South forks of the river to Waconda Lake instead of the outlet flow (lake) chloride content for the same date does not change the outlet release calculation significantly. Thus, limitations imposed on bypassing natural flow entering Waconda Lake are not expected to be an important consideration in the release of lake water to dilute high chloride content in the Solomon River at Beloit. It is recommended that a verifiably accurate measurement of chloride concentration be obtained for the Solomon River at Beloit before any dilution release is made from the lake.
The sulfate content of the Solomon River at Beloit has often exceeded the secondary drinking water limit of 250 mg/L during the last decade. The range and average sulfate concentration in the dam outlet flow that represents Waconda Lake water was 265-304 mg/L and 284 mg/L, respectively, during 2002-2003. Releases of lake water cannot be used to achieve dilution of the sulfate level in the river at Beloit based on these conditions.
Evaluation of long-term records indicate that chloride and sulfate concentrations have been increasing at varying rates in the North Fork Solomon River at Portis, the South Fork Solomon River at or near Osborne, Waconda Lake, and the Solomon River below Glen Elder Dam and at Beloit. Short-term fluctuations in flow due to climatic variations produce a larger magnitude in the chloride and sulfate changes than the long-term trends. However, the long-term trend of increasing sulfate content has caused Solomon River water at Beloit to exceed the secondary drinking water limit much more often than in the past. If the long-term increase in chloride concentration continues at a similar rate as in the past, the frequency at which the river water at Beloit exceeds the secondary drinking water limit could also increase, although substantial chloride loading would need to occur for Waconda Lake to reach the secondary criterion.
Kansas Geological Survey, Geohydrology
for Diluting Saline Water
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Placed online Jan. 18, 2006