|Cumberland: Larger Than It Seems|
Cumberland, located on Vancouver Island, near Courtenay, is a historical town, founded before the turn of the nineteenth century to provide housing for workers in the coalfields. Its storm and sanitary sewers are combined. Treatment to secondary standards is by lagoons, one aerated and the second facultative. This paper will review the plans and expected water quality improvements of their proposed constructed wetland.
ANIMAL BIOMASS Before European agriculturists began farming in the North America, there was about the same order of magnitude of animal biomass as there is now. Bison by the tens if not hundreds of millions, passenger pigeons with single flocks containing billions of individuals, several hundred million beaver, and countless other birds and mammals, shaped the landscape, and shaped the ecology of coastal waters.
The difference between then and now is formerly the biomass was dispersed though out the continent. Their “droppings” were widely dispersed, and were filtered through forest and grassland soils, with virtually the last vestiges of contaminants removed by the once extensive wetlands. Now much of the population lives near rivers and in costal areas in large agglomerations. There is as much cattle manure produced in the San Fernando Valley by dairy herds as by humans in the Los Angeles area. A constructed wetland is proposed to treat the entire San Fernando River, which is highly enriched by the dairy cattle.
RESULTING CHANGES IN COASTAL ECOLOGY Wastes are concentrated consequently and no longer filtered as extensively as once. The results, among other things, have been an unprecedented change in the ecology of coastal waters. “Red Tides” once restricted to high latitudes on the Atlantic and Pacific coasts are now virtually circum-continent.
In a report just release by the National Academies excessive nutrients from a plethora of sources is sited as the root cause of the changes that are occurring. In another recent report, the National Coastal Commission Report rates the overall conditions of the coastal waters of the US as fair to poor. “Half of all the male fish in lowland rivers of Britain are developing female characteristics as a result of pollution, alarming new official research suggests.” What we are doing is not sustainable. If we do not reduce markedly the quantity of nutrients going into coastal waters we can expect the demise of species after species, and continuing human health problems. The solution to pollution is NOT dilution it is treatment with water reuse to remove contaminants.
Cumberland is in the process of planning on doing something about their wastewaters. Proposed is a state of the art and science constructed wetland. The present conceptual plan, just completed, calls for a 13 ha to be utilized in a free water surface enhancement wetland.
Wetland Types: The most common constructed wetlands are treatment and enhancement.
A treatment wetland is a shallow pond filled with emergent vegetation with no open water. Particulate matter removal and denitrification are the main functional properties.
An enhancement wetland contains both shallow, emergent vegetation and deeper, open water. Designed and constructed to mimic nature as closely as possible, while avoiding short-circuiting, enhancement wetlands maximize diversity of water depths, plant species, and habitat. Consequently they better establish conditions similar to those existent in natural wetlands, nature’s way of contaminant removal. Open water serves to oxygenate, thus nitrify ammonia. Enhancement wetlands also can have a plethora of ancillary benefits, such as passive recreation, and wildlife habitat creation.
Removal Of BOD5, TSS, Nitrogen, And Phosphorus In Constructed Wetlands
The most commonly measured estimates of organic pollution loadings that track complexed compounds are: five-day Biological Oxygen Demand (BOD5), Total suspended solids (TSS), and nitrogen species. Properly designed constructed wetlands remove the above contaminants superbly well Stowell et al., 1981, Tchobanoglous, 1987, Gearheart, et al., 1992. Phosphorus, being an element, requires a far larger area for continual net removals to occur Gearheart, 1993. Consequently, wetland designed for phosphorus removal will be amply sized to break down complexed carbonaceous and nitrogenous compounds. Constructed wetlands remove not only the contaminants we measure, but also the ones we do not Gearheart, pers. com., 2001.
Removal of Phosphorus by Wetlands Richardson and Qian, 2000, developed a phosphorus mass-loading model derived from the North American Wetland Data Base (NAWDB). Their work on naturally existing, low nutrient input wetlands reveals about 1 g of P per m2 year –1 can be introduced into a wetland without altering either ecosystem structure or function. From their cross-sectional analysis they proposed the "One Gram Assimilative Capacity Rule" for P loadings within natural freshwater wetlands for long-term storage of P, and maintenance of community structure and function, and low P effluent concentrations. Much higher levels can be accommodated, although phosphorus secretions can be expected. Richardson, Walbridge, and Burns 1988, estimate certain wetland soils have a phosphorus assimilative capacity of up to 42 g m2 yr -1. In studies of a constructed wetland very heavily loaded with phosphorus, Gale, Reddy and Graetz, 1994, found anoxic organic soils assimilated 1.4 g P kg -1, and aerobic wetland soils, 1.8 g P kg-1.
Seasonality of Phosphorus Removal by Wetlands While a net nutrient sink, wetlands retain and secrete phosphorus according to a complex array of climatological and biological factors. The removal of phosphorus from wetlands is seasonal and intermittent, Gearheart, 1993. Wetlands remove phosphorus principally by precipitation and adsorption to sediments, with secondary mechanisms of plant intake and burial in sediments Tchobanoglous, 1987. Generally, wetlands retain phosphorus during the summer growing season and secrete it during the winter when environmental sensitivity is at a minimum, Stowell, et al., 1981.
Background Phosphorus Levels Virtually all wetlands studied are in some way unique. One essential measurement required to set seasonal discharge levels are background levels in non-anthropogenically impacted, naturally occurring waters in the area of the discharge. Background levels are an expression of how a natural wetlands function, and so how the watershed evolved. Maple Lake, located in the immediate vicinity of the project, is the best example of a local, natural wetland. It is a geologically mature wetland with peat deposits near Cumberland Road being in excess of 20 meters. There are few anthropogenic influences, although several commercial properties are located north of Cumberland Road. While Mimulus personnel had one sample point, it was above where the main quantity of water drains from the subject area emanating from the west side area, probably originating from underneath Cumberland Road, west of the culvert Mimulus, 2001. Data is insufficient to establish background levels. The July and August samples were at 0.17 mg/L total P.
Wetland Design Features Being Considered
Lagoons The existing lagoons are to be retained. Water quality test indicate they are presently functioning quite well. See the Kerr Wood Leidal report 2002 for details on the suggested improvements on the lagoon as population increases. The constructed wetlands are to be placed downstream and to the north of the existing lagoons.
Wetland The wetland system conceptually designed and modeled for Cumberland has four treatment cells (A-D) for wastewater polishing, a restored marsh (Cell E) to be used for storing stormwater to blend with the wastewater during critical times, and a small dilution cell (Cell F) for blending, directing, and releasing flows. Maple Lake Creek (Cell G) is restored to approximate what it once could have functioned as. It will be located to the east of the treatment cells. These cells are shown on the Cumberland Constructed Wetlands Schematic and briefly described below. The cells form a unified, complex ecological unit composed of small and large ponds with islands, marshes, a stream, and riparian areas. Treatment cells will be hydraulically separate from the stormwater cell and restored riparian areas. While Maple Lake is seasonally secreting phosphorous, flows should be kept separate. Maximum flexibility for directing flows is inherent in the design.
Operational Overview The operation of the wetland system is designed to keep the release of water into Maple Lake Creek low in phosphorous during critical summer time periods, with phosphorus releases during winter when other factors limit primary productivity. For wetlands to remove phosphorus, its necessary to bypass the wetlands or dilute the out-flow (with stored stormwater) during certain the months when the wetlands do not remove phosphorus efficiently. It is desirable to push as much phosphorus out of the system during high flows (non-critical times) so it will not re-emerge from the wetland during low flow periods. The less phosphorus that is put into the wetland, the less will emerge, and the higher the removal percentage will be.
Complexed Compounds Removal Cells A and B are wetland cells designed to remove suspended solids, BOD, coliform bacteria, nitrogen, and heavy metals. The current best design for this type of wetland includes zones of emergent marsh plants interspersed with zones of deeper, open water. These two cells will also provide some phosphorous treatment. To work efficiently these cells must have strict vegetation and topography specifications.
Additional Phosphorus Retention Cells C and D are designed to remove maximum amounts of phosphorous. These cells provide additional detention time and should be planted with species less likely to re-introduce phosphorus. Stormwater can be introduced into the north end of these cells, possibly providing fine sediments, which phosphorus adsorbs onto, annually restoring some of the treatment capacity of the cells. These cells also have strict vegetation and topography specifications to work efficiently.
Stormwater Storage Cell E is designed to store water from Maple Lake Creek and to provide habitat diversity and public use possibilities. This area will not be loaded with phosphorous emanating from the lagoons. It is expected MLC water will be suitable for blending with wetland water before release to keep phosphorous concentration in the released water as low as possible. Vegetation and topography can be of many forms in this cell and so this is a good location for marsh habitat consistent with the restored Maple Lake Creek area. Islands will be inherent in the design for excavated soil storage.
Estimated Phosphorus Concentrations A spreadsheet model of the proposed system has been developed to estimate the phosphorous concentrations of the water as it flows through the various cells. The scenario employed is a year of typical rainfall and average temperatures for a population of 5,000. See Table 3
Seasonality Wastewater flows through cells A-D for the seven summer months, the time marshes best remove phosphorous. During March, November and December, by using some dilution from Cell E, a 1-mg/L criterion is met with discharge directly to Maple Lake Creek. During January and February, Cells C and D are by passed, allowing these cells to be filled with stormwater to enhance their phosphorous treatment efficiency at other more critical times.
Phosphorus Removal Model Phosphorous removal in the wetlands was estimated by separating the removal capacity into two elements, biological removal and precipitation-adsorption removal.
Projected Phosphorous Concentrations Phosphorus levels at the wetland discharge point to Maple Lake Creek are kept below 1 mg/l in an average year. Concentrations are highest (nearly 1 mg/l) during the months of high flow and low biological activity, and are lowest (0.48 mg/l) during June, a time of high biological activity.