2016 Monitoring Report; Mississippi River Tributaries

Transcription

1 2016 Monitoring Report; Mississippi River Tributaries SWWD s East Mississippi watershed (Map 1) is named after the former East Mississippi Watershed Management Organization which managed the area prior to consolidating with SWWD. The East Mississippi watershed consists of Newport, St. Paul Park, Grey Cloud Island Township, and small portions of the Cities of Woodbury and Cottage Grove. The watershed lies along the Eastern bank of the Mississippi River on the Western edge of SWWD and drains to the river through an extensive and aging storm sewer network. While the East Mississippi watershed primarily consists of sub-surface drainage, the watershed also includes La Lake which is currently land-locked. The Central Ravine watershed drains a large portion of Cottage Grove. The 2,720 acre watershed is largely built out with an extensive storm sewer system that was constructed prior to current Watershed District standards. Map 1: Mississippi River Monitoring Sites This report summarizes the 2016 monitoring results for the East Mississippi and Central Ravine watersheds which collectively represent areas tributary to the Mississippi River. SWWD s monitoring programs are based on a Regional Assessment approach. By following a regional approach, monitoring is focused on key resources and watershed outlets throughout the District. Regional Assessment Locations for the East Mississippi and Central Ravine watersheds are La Lake, and the Newport, St. Paul Park, and Central Draw storm sewer locations. Lake water quality is monitored through participation in the Metropolitan Council s Citizen Assisted Monitoring Program (CAMP). Regional Assessment sites consist of automated sampling equipment that collects data necessary to calculate annual loading rates of various pollutants monitoring results for La Lake and the Newport, St. Paul Park, and Central Ravine regional assessment sites are presented individually below.

2 La Lake Map 2: La Lake DNR ID # Municipality: Woodbury Surface Area: 45 Acres Watershed Area: 81 Acres Mean Depth: 6 feet Maximum Depth: 10 feet SWWD Maximum Allowable Phosphorus Load: 1.65 lbs/ac/yr SWWD Trophic State Index (TSI) Goal: La Lake is a land-locked shallow basin within the East Mississippi watershed. The less than 2- to-1 ratio of drainage area to lake surface area implies that inputs to the lake will be relatively straightforward to manage. The lake s drainage area is predominantly undeveloped with a mix of natural habitat areas and some agricultural land use. The watershed is primarily publicly owned although a portion does remain in private ownership. Currently, however, no storm sewer outfalls discharge to the lake and previous existence of septic systems for the few residences around the lake is likely of little concern to the lake. This lake is classified as a wetland by the Minnesota DNR, and has been used in the past by MnDNR Fisheries as a walleye rearing pond. SWWD sets a TSI goal of for La Lake, Figure 1: La Lake Surface Elevation corresponding to an allowable watershed Total Phosphorus (TP) loading rate of 1.65 lbs/ac/yr. SWWD s goal encompasses MnPCA water quality standards for shallow lakes in the region. La Lake displays surface elevation trends (Figure 1) typical for land locked basins. With no outlet, the surface elevation rises during years of high precipitation and slowly falls during years with less precipitation. It remains high following 2016 which saw record precipitation. In-lake TP shows significant year to year differences (p < 0.01) although there is no consistent upward or downward trend (Figure 2). Rather, TP spikes with lower lake levels, perhaps indicating a stronger effect from internal loading during low water periods. In contrast, Secchi Transparency has routinely met or exceeded both SWWD and State benchmarks (Figure 3). There are significant year to year differences (p < 0.01) in Secchi Transparency; however there is not a consistent upward or downward trend. Rather, it appears that the lake goes through alternating clear and turbid phases. Year to year differences in chlorophyll a (Figure 4) are not significant (p = 0.32). Overall, La Lake typically grades out near the middle for metro lakes as determined by the Metropolitan Council (Table 1). Though swimming is often impaired during the summer, both secchi transparency and chlorophyll a (measure of algae) are generally better 2

3 than expected based on TP concentrations. This imbalance is likely indicative of an extensive vegetation community. A vegetation survey of La Lake was completed in % of the lake is vegetated with 17% of lake having vegetation to the lake surface. One non-native, invasive species was present purple loosestrife. The survey will serve as baseline against which to measure future changes. Figure 2: In-lake TP Concentration for La Lake TP SWWD TP Goal MPCA TP Standard TP (mg/l) Figure 3: Secchi Transparency for La Lake Secchi Transparency (m) Secchi Transparency SWWD Secchi Goal MPCA Secchi Standard 3

4 Figure 4: In-lake Chlorophyll a Concentration for La Lake Chlorophyll a (ug/l) Chlorophyll a SWWD Chlorophyll a Goal Table 1: Annual Lake Grades for La Lake Parameter Tophic Status Total Phosphorus 61; Eutrophic Chlorophyll 52; Eutrophic Secchi Transparency 56; Mesotrophic Lake Grade C C D D C D D D D C D C D D D D D D F C C C C B A B C B C C C B C B B C D B C C B C B C B B C B C C B C C C C B D C C D C C C B C C C C C Overall Eutrophic C B C C B C C C C C C C C D C C C C D C C C C Note: Lake Grades are based on comparison with other lakes in the Minneapolis-St. Paul metropolitan area. Criteria for assigning lake grades are established by the Metropolitan Council. 4

5 Newport Municipality: Newport Watershed Area: 300 Acres SWWD Maximum Allowable Phosphorus Load: 0.22 lbs/ac/yr The Newport site is on a tributary to the Mississippi River through which stormwater runoff from a large portion of the City flows. This station serves as a regional assessment location for the City of Newport and will help develop baseline water quality and quantity data for runoff flowing into the Mississippi River. Annual loading data for previous years is summarized in Table 2. No significant trends have emerged since monitoring began in 2006, except for chloride load which has increased dramatically, reflecting de-icing operations. Additionally, TSS loading rates at the Newport site generally meets proposed TSS loads ( lbs/ac/yr) for the draft South Metro Mississippi River TSS TMDL. Other pollutants primarily copper, lead, and zinc are routinely elevated. High levels of heavy metals most likely reflects a relative lack of treatment in the existing stormwater system which was largely built prior to current standards. Any future redevelopment would be subject to more stringent stormwater standards and would likely make a significant reduction in the pollutant levels at the Newport monitoring site. Water quality data is available through the SWWD office. Map 3: Newport Regional Assessment Site Table 2: Loading Summary for Newport Year Precipitation (in) Runoff Yield (cu ft runoff/in TP (lbs) TP Yield TSS (lbs) TSS Yield (lbs/ in (lbs) Yield , , , , , , , , , , , , , , , , , , , , ,829 1, , , ,099 1, , , ,737 1, , , ,030 1, , , ,690 1,136 Note: Reported loads and rates are scaled to reflect an April through October growing season. 5

6 St. Paul Park Municipality: Cottage Grove Watershed Area: SWWD Maximum Allowable Phosphorus Load: 0.22 lbs/ac/yr The St. Paul Park site (Map 3) monitors discharge to the Mississippi River. This station serves as a regional assessment location for the City of St. Paul Park and will help develop baseline water quality and quantity data for runoff flowing into the Mississippi River. Annual loading data for St. Paul Park is summarized in Table 3. No consistent trends have emerged since monitoring began in Runoff yields are highly variable, potentially indicating illicit discharge or inflow infiltration of the storm sewer network. Map 4: St. Paul Park Regional Assessment Location In addition to TP and TSS, SWWD monitors runoff for various pollutants, several of which (copper, lead, and zinc) are routinely elevated. Routinely high levels of heavy metals most likely reflects aging infrastructure. Any future re-development would be subject to more stringent stormwater standards and would likely make a significant reduction in the pollutant levels at the St. Paul Park monitoring site. Water quality data is available through the SWWD office. Table 2: Annual Loading Summary for the St. Paul Park Regional Assessment Location Year Precipitation (in) Runoff Yield (cu ft runoff/in TP (lbs) TP Yield TSS (lbs) TSS Yield (lbs) Yield , , , , , , , , , , , ,452 3, , ,600 1, , , , , , , , , , Note: Reported loads and rates are scaled to reflect an April through October growing season. 6

7 Central Ravine Municipality: Cottage Grove Watershed Area: 2,720 Acres SWWD Maximum Allowable Phosphorus Load: 0.22 lbs/ac/yr The Central Ravine regional assessment monitoring station (Map 5) was established in 2009 and is used to assess the 2,720 acre Central Draw watershed. The Central Draw watershed is largely built out with an extensive storm sewer system that was constructed prior to current Watershed District standards. The primary purpose of the Central Ravine site is to assess the water quality of water draining out of the Central Ravine watershed and evaluate the effectiveness of various water quality improvement projects planned Map 5: Central Ravine Monitoring Site for the Central Ravine. Ultimately, this site will drain additional runoff flowing out of SWWD s Northern Watershed following completion of SWWD s planned Central Draw Emergency Overflow. Water flowing through the Central Ravine station discharges into a large wetland complex before ultimately draining to the Mississippi River. Annual loading data is summarized in Table 4. Runoff, total phosphorus, and total suspended solids are all relatively stable after a number of years with elevated levels. Any future redevelopment would be subject to more stringent stormwater standards and would likely make a significant reduction in the pollutant levels at the Central Ravine monitoring site. Table 4: Annual Loading Summary for the Central Ravine Regional Assessment Location Year Precipitation (in) Runoff Yield (cu ft runoff/in TP (lbs) TP Yield TSS (lbs) TSS Yield (lbs) Yield (lbs/ in , ,500 2,164 7, , ,424 7,601 10, , ,460 16,764 6, , ,869 15,623 4, , ,442 17,967 5, , ,340 7,223 21, , ,696 5,059 8, , ,622 2, Note: Reported loads and rates are scaled to reflect an April through October growing season. 7