Hypolimnetic withdrawl from a shallow, eutrophic lake

• Main Author: Macdonald, Ronald H.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4201

Hypolimnetic withdrawal is a technique for reducing trophic status in a high productivity, or eutrophic, lake. This thesis presents a case study of a hypolimnetic withdrawal application to a shallow lake in the B.C. interior. The installation demonstrates an appropriate technology for lake restoration suitable for recreational properties where the population base is small, or residents may not be present continuously. The polymictic nature of the lake is evident from daily average water temperatures. Partial destratifications occur within a day and complete de-stratifications occur over a period of a few days. The difference in stratification between the two observed summers highlights the significance of inter-year meteorological variation in determining lake response. Observed responses to wind forcing included surface mixing, isotherm tilting and seiching. A typical summertime seiche period of 10 hours was observed during 1994. Theoretical predictions of the seiche period using two-layer stratification and linear stratification estimates provide approximate upper and lower bounds for the observed response, as determined from spectral analysis. Wind events and lake thermal response can be parameterized by the Wedderburn number. Substantial upwelling and destratification was observed when the Wedderburn number was between 1 and 2. Higher Wedderburn numbers resulted in seiching responses. A small deep area provides a quiescent region in the lake that is thermally stratified from the main water column. This area was low in oxygen through the entire 1994 withdrawal season and had elevated concentrations of phosphorus, iron, and manganese indicating that this area may not mix during spring turnover, or that oxygen depletion rates are sufficient to result in anoxia only four weeks after ice off. The chemistry of the lake agreed with our expectation of an anoxic, internally loaded system. Following the depletion of oxygen, concentrations of phosphorus, iron, and manganese increased in the lower levels of the lake. This build-up was sustained for two weeks without mixing to the surface due to thermal stratification. Subsequent surface cooling resulted in partial mixing. The resulting circulation of nutrients initiated an algae bloom. During the first year of operation, a total of 0.63 m of lake (10% of volume) was removed through the withdrawal. This resulted in the export of about 30 kg of total phosphorus. Comparison of the maximum phosphorus observed in the lake for several years indicates that relative to other years, the phosphorus in Chain Lake was about the same or slightly lower than in other years. Rough calculations indicate that the withdrawal may achieve a net phosphorus export of 5-20 kg/yr, depleting the sediment phosphorus pool in times scales of centuries. A correlation between the daily average air temperature in Princeton, and the lake stability shows that lake stratifications are coincident with long periods of above average temperatures in Princeton. The correlation permits use of the long term meteorological record to interpret the frequency distribution of lake stratifications. The results indicate that the stratification observed in 1994 was a once in 7 seven year event in terms of the length of stratification. Monitoring evaluated the environmental impacts on Chain Lake and Hayes Creek. The withdrawal is expected to have a negligible impact on the thermal structure of the lake. Dissolved oxygen deficits were observed throughout the summer at the withdrawal and an attached fountain was able to increase the dissolved oxygen by about 1.5 mg/L. High levels of ammonia, iron, and manganese were observed during the July stratification. The ammonia was quickly consumed, or lost in Hayes Creek, while iron and manganese exhibited an attenuation in which the peak levels were reduced, and the duration increased. After 5 weeks of stratification, sulfide was detected on August 8. Overall, the environmental impacts of the withdrawal are localized within the area of the Hayes Creek between the Fountain and the Jellicoe Road station (about 500 m).