02-22-2024 PRF Packet - Laserfiche WebLink

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January 26,2024 Don Peterson Page 13 of 21 Reference: Water Quality Assessment of Silver View Pond,Mounds View,MN the surface in late summer to open the surface of the water to the atmosphere as summertime DO drops considerably when the surface is covered by floating-leaved vegetation. SEDIMENT MANAGEMENT Our field assessments indicated accumulation of sediment around some of the outlet structures. We recommend removal of sediment plumes at these structures to original pond depths, as determined in the bathymetric survey performed as part of the June 2023 Mounds View Stormwater Pond Assessment. NUTRIENT REDUCTION Internal load reduction There are primarily four different strategies for alum applications to lakes, which can be applied individually or in some combination to achieve desired management goals. The strategies are described below. • Water column stripping: This strategy targets the phosphorus in the water column but does not account for the mass of phosphorus in the sediments. It is ineffective for capturing additional external loads following an initial alum application, as the material will settle to the sediments within 24-48 hours. • Sediment phosphorus inactivation: This strategy targets the mass of phosphorus in the sediments that is subject to mobilization and diffusion from the sediments to the water column. Upon application, some alum serves to"strip the water column"of phosphorus as well. Consequently, the water column needs for phosphorus binding are included in the dosing calculations. • Phosphorus interception: This strategy involves addition of alum (or an alum-based coagulant)to a tributary inflow to reduce the external phosphorus load before entering the lake. • Maintenance treatments: Maintenance dosing may target the water column or the sediments depending on the goal. Maintenance treatments are particularly useful when a sediment inactivation treatment has been applied, but continued external loading sustains high water column phosphorus levels and accumulation of phosphorus in the sediments overlying the floc layer. Maintenance doses function to bind with phosphorus that has entered the lake since the previous application. We recommend a pilot alum application that employs a water column stripping approach where alum is applied to the entire pond in the spring following ice-out but prior to growth of dense abundant vegetation. This would serve to bind phosphorus that is in the water column at the time of application, which will settle to the sediments in the form of an aluminum phosphate floc that is unavailable for algal uptake. Once settled to the sediments, the alum floc would capture some phosphorus in the sediments until the binding site of the alum has been occupied. Water samples for phosphorus and chlorophyll-a analysis would be collected biweekly from Silver View Pond to evaluate success of the pilot application. Depending on the pond's response to the treatment (as evidenced by phosphorus and chlorophyll-a), the strategy may need to be adaptively managed the following year. For example, an application prior to fall ice-on may provide Design with community in mind