2002 Draft Natural Resources Management

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Watershed and Lake Response Modeling Watershed Model. As in the case of Langton, the PONDNET was used to estimate runoff generated phosphorus entering Bennett. For more information on the PONDNET model refer to the Langton Lake section on modeling. Table 4 in Appendix A shows results from the PONDNET model for Bennett Lake. By model estimates, Bennett receives an average annual phosphorus load of about 540 pounds from its watershed. Total phosphorus loading per acre of lake is estimated at about 20 lbs (rnore than double the loading per acre for Langton Lake) and reflects much higher inputs than the lake can reasonably be expected to absorb and stay healthy. Phosphorus reznoval efficiencies for ponds in the Bennett watershed range from 37°Io to 56%. Ponds WP-1 and BL-4 serve important roles in treatin� stormwater that eventually reaches Bennett Lake. Modification to increase the standing water volume of ponds in the watershed may be a cost-effective strategy to enhance the stormwater treatment funetions of these ponds and decrease watershed phosphorus loads reachin� Bennett Lake. Improvement efforts should focus on those ponds that receive runoff from iarge draina�e areas and whose model estimated removal efficiencics are low {less than 40-50%). Lake Rcsponse Models: Totai phosphorus and water loading rates from PONDNET aiong with thc actual measured growing season mean pl�osphorus level was used as input to the WiLMS model in order to estimate in-lake phasphonis concentrations (Appendix A, Table 6). For more information on the WiLMS modcl rcfer to thc Lan�ton section on rr�odeiing. The Rechow, 1979 ,_.e....e.�� .z.7.�1 ..._�.....7..,�E .L„�. 4.,.,.. r.� F,�... �l..,. .],..,, --.:�4 ,.....,,.,..�.-.,] .,..,.,... ..1.,.,..-,L,,....... --.�-_�.zat_�..,.�.. -.r �vli'viui iiiViivi �11VYSL1Vii Llll� V\+JL aa� 1Vl ili'v LL[ALG •��a 111liCLJl.l1'vw lllVtl�l tl1�VJEJl1V11AJ vviiv'viiiiwiiVli va 1�� U�1. In a fashio� similar t� Langton, hypothetical changes in phosphorus concentrations resulting fro�n percent reduction in phasphorus loadings were calculated. In the case of Bennett Lake, a 4Q% reduction in phosphorus loads would be needed in order to bring in-Iake water quality up to Grade C on. the Met Council's scale. This is a very large reduction and is Iikely to be achieved only with significant and expensive infrastructure changes and intensive mana�ement activities in the lake itself. City of Rosei�ille 51 Parlts Nnttu•al Resor�rce Mctrragerrient