|
|
Habitat Degradation |
A variety of activities centered in or around water resources can have a detrimental impact on the habitat and water quality associated with those resources. The impacts from such activities on water quality can be two-fold; impacted areas can act as sources of pollutants, and the prior ability of impacted areas to provide water quality improvements to throughflows can be compromised or eliminated. Measures can be taken to mitigate these impacts or to restore the affected habitat, restoring water quality in the process. Measures vary with the type of impact, but a sampling of the more common impacts and mitigative BMPs is provided below. Sources of detailed information on the restoration of wetlands, riparian areas, streambanks, and streambeds are provided in the references at the end of this section.
WETLANDS AND RIPARIAN AREAS
Wetlands and riparian areas perform a number of water quality functions in addition to other functions. Under natural and artificial loading conditions, they can remove sediment and chemicals sorbed to sediment, nutrients, metals, organic matter, toxic chemicals, and other contaminants. For a more detailed discussion of wetland and riparian area functions and values, click here.
Wetlands are recognized as dynamic ecosystems, undergoing changes on daily, seasonal, and long-term bases. We are interested here, however, in discussing amelioration of the undesirable changes wrought by human disturbances. Many activities conducted by humans can compromise or eliminate the ability of wetlands and riparian areas to perform some or all of their water quality functions. Recovery of those functions would involve reversing such impacts. The following discussion deals with wetland and riparian area BMPs in a nonspecific sense, discussing impacts to and restoration of wetlands in general. More specific discussion will necessarily apply to some wetland types and not to others. For an evaluation of the relevance of recommendations to your region's ecotypes or to specific systems, local wetlands experts should be consulted. Note that the following discussion refers to wetlands and riparian areas simply as wetlands for brevity. Most of the discussion can apply to both types of habitats.
Reestablish Hydrology
The most important element of wetland function, and perhaps the most frequently altered, is hydrology. A wetland's hydrologic regime can be altered in a few basic ways. One of those ways is draining, or lowering the elevation at which the system is able to discharge. Probably the most common method of draining involves ditching the wetland's outfall. In many instances, this can also be one of the easiest impacts to remedy, requiring only a determination of the pre-disturbance surface overflow elevation or the closest level that competing interests will allow, followed by blocking of the outfall ditch, allowing other wetland functions to return as the pre-disturbance hydrologic regime reestablishes. In the case of the channelization of a stream or river which serves as the outfall for contiguous floodplain wetlands, the solution can be much more involved. Channelization of streams often has occurred to provide flood relief for development within the floodplain during high flow events. Restoration of such streams would require gaining control of or acquiring the affected floodplain areas or portions of them based on modeling results, followed by reestablishment of channel sinuosity with in-stream blockages and reestablishment of bed elevation with grade control structures. Other methods of draining wetlands amount to variations on the theme of ditching. Wetlands can be drained by ditches which disrupt a lens or impermeable horizon beneath the system for some distance. In such cases, restoration may require filling of the length of the intercepted unit with clay or other low permeability material. Adjacent ditching or nearby large-scale excavation can act to draw wetlands down more rapidly, shortening the hydroperiod's duration and decreasing its amplitude. Construction of a hydraulic barrier wall by trenching and filling with clay or other material may by effective in such cases, particularly if the wall is tied into some confining unit in the soil. Actions to minimize the drawdown source may also be necessary, such as relocation in the case of the adjacent ditch or cessation or minimization of dewatering activities in the case of a large-scale excavation. One other form of drainage occurs in agricultural settings and involves the direct underdraining of the wetland with drain tiles. Plugging of the drain tiles' outfall points can effectively stop their drawdown impacts, and the tiles are likely to sediment in over time.
A second basic means of altering wetland hydrology is by diverting the inflows, either surface or subsurface, to wetlands. In rural and agricultural settings, interceptor ditches skirting the periphery of wetlands are neutralized most effectively by blocking them at intervals along their path or completely filling them. If only a single well-placed ditch block is used, the upper elevations of the wetlands may not recover and the entire wetland may be denied important dry weather inflows which can affect the ability to reestablish the desired biotic communities. These issues vary greatly with wetland ecoregion and type, and may not be of concern in many systems. A wetland's contributing area may also be diverted in an agricultural setting through the use of drain tiles under a field or a surface ditching network which routes the discharge to another, often lower point in the landscape. Once control of the land or landowner cooperation is obtained, ditches and drain tiles can be plugged. In urban areas, diversion of inflows is common and takes the form of impervious surfaces, which intercept rainfall and preclude its natural infiltration into the soils and movement into wetlands as subsurface flow. Surface runoff which naturally collects in wetlands is intercepted by impervious surfaces and, along with the rainfall that would otherwise infiltrate and move as subsurface flow to wetlands, it is routed either to stormwater systems which typically discharge downgradient or directly to convenient, lower-elevation outfalls. Restoration of such urban wetland systems can be complicated, and may require pumping of stormwater or redirection of flows, which may only be achievable for small portions of contributing area or larger areas as land uses change. Stormwater engineers should be consulted for advice on specific cases.
The last basic means of altering wetland hydrology is essentially by drawing the water out from underneath a wetland. Local or regional drawdown of the surficial aquifer can occur as a result of industrial activity or mining involving either intentional or consequential drawdown of the water table, as a result of overpumping by wellfields, or as a result of channelizing of regional surface water drainage networks for flood control and development purposes. Such regional drawdowns typically must be addressed by working with the drawdown source(s) to minimize the drawdown magnitude. In addition, if the drawdown is relatively localized and active for a relatively limited time period, some form of continuous groundwater surcharge zone can be established between the activity and the impacted system to offset the ongoing depletion. If the drawdown activity is relatively permanent and affects only one or a small number of wetlands, a more permanent barrier such as a clay or synthetic groundwater barrier wall can be installed. Such a barrier is more effective if it ties into an impermeable soil horizon. If not, it may be most effective to use a combination of BMPs such as minimization of the hydraulic head differential (decreasing the drawdown), construction of a barrier wall, and surcharging as necessary on the upgradient side of the barrier.
Minimize Other Disturbances
While alteration of hydrology is probably the most common impact to wetlands and riparian areas, other disturbances can be equally damaging. Wetlands being considered for restoration should be assessed for the potential occurrence of and ability to ameliorate or remove other ongoing disturbances, outside of natural disturbance processes, in addition to hydrologic alterations. Those disturbances typically take the form of significantly accelerated loading of pollutants carried by inflows to the wetland. The most damaging of these may be accelerated sediment deposition and nutrient loading. If signs of unnatural sediment accumulation (deltas of bare sediment, buried litter, etc.) or nutrient loading (eg., shift in edge species composition to opportunistic varieties without signs of physical disturbance) are found within the system, the wetland's contributing area should be evaluated. The ability to either make changes to land use practices at the source(s) and/or to install BMPs to capture pollutants during transport should be considered. To access descriptions of applicable BMPs through an index of source categories, please click here . An appropriate goal of restoration may well be to rehabilitate a wetland so that it can provide better pollutant removal. The intent of investigating upgradient BMPs, then, is to provide for sufficient pre-treatment to ensure the integrity and continued ability of the wetland to perform its desired functions, as a "car" in the water quality improvement "train".
Other disturbances often involve direct disturbance through logging, filling, excavation, peat mining, farming and animal access practices, and enhanced fire frequency. These impacts are more discrete in nature, and the likelihood of future occurrences should be investigated and minimized.
Restore Native Plant Communities
Reestablishment of degraded or displaced native plant communities can provide water quality benefits in some cases. Hydrology is the most important element not only of wetland function in general, but also of maintaining or establishing viable, diverse, native plant communities in wetlands. It is not advisable to attempt reestablishment of lost flora where compromised hydrology has not been reestablished. If hydrology can be fully reestablished to pre-disturbance form, then pre-disturbance plant communities will likely follow independently given sufficient time. The time needed, however, is highly variable and can appear, in practical terms, indefinite. Therefore, it is often desirable to set the successional process forward by constructing the core elements of the desired flora and performing maintenance to further ensure their establishment. Such restoration work is particularly valuable when forested wetlands were lost and a recalcitrant herbaceous community has secured the site in their place. In situations where pre-disturbance hydrology cannot be fully reestablished, appropriate native flora can be established based on the hydrologic regime expected for the system. Forested systems can provide greater sequestration of nutrients in above- and below-ground biomass for long-term storage than herbaceous and shrub systems, and can be harvested periodically for profit and to remove those nutrients from the system. Further, their greater below-ground biomass can be permanently buried in the litter compartment. In riparian systems particularly, nonleguminous hardwood trees are the most effective vegetation for nitrate removal from subsurface flows, having deeper root penetration than other vegetation. A mixture of trees, shrubs, and herbaceous cover may provide the fullest nitrate removal in riparian areas. Dense-culmed herbaceous cover is most effective for filtration of suspended solids. Frequent maintenance during the initial establishment period to remove undesirable vegetation and regular monitoring and maintenance throughout the establishment of the desired plant community is fundamental to any restoration plan.
STREAMBANKS AND STREAMBEDS
For a discussion of STREAMBANK BMPs, click here. For a discussion of STREAMBED BMPs, click here.
Cairns, J., K.L. Dickson, and E.E. Herricks (eds.), 1977. Recovery and Restoration of Damaged Ecosystems. University Press of Virginia, Charlottesville, Virginia. 531pp.
Fisk, D.W. (ed.), 1989. Wetlands: Concerns and Successes. Publication no. TPS-89-3. American Water Resources Association, Bethesda, Maryland. 568pp.
Holman, R.E., and W.S. Childres, 1995. Wetland Restoration and Creation: Development of a Handbook Covering Six Coastal Wetland Types. Report no. 289, Water Resources Research Institute of the University of North Carolina, Raleigh, NC.
Kentula, M.E., R.P. Brooks, S.E. Gwin, C.C. Holland, A.D. Sherman, and J.C. Sifneos, 1992. Wetlands: An Approach to Improving Decision Making in Wetland Restoration and Creation. Island Press, Covelo, California. 151pp Kusler, J.A., and M.E. Kentula (eds.), 1990. Wetland Creation and Restoration: The Status of the Science. Island Press, Washington, DC. 594pp.
Lewis, R.L., 1982. Creation and Restoration of Coastal Plant Communities. CRC Press, Inc. Boca Raton, Florida. 219pp.
Lewis, R.L., 1994. Enhancement, Restoration and Creation of Coastal Wetlands. Pages 167-192. In D.M. Kent(ed.), Applied Wetlands Science and Technology. Lewis Publishers, CRC Press, Inc. Boca Raton, Florida. Mitsch, W.J., and J.G. Gosselink, 1986. Wetlands. Van Nostrand Reinhold, New York, NY. 539pp.
MWCOG, 1992. Watershed Restoration Sourcebook. Publication no. 92702. Metropolitan Washington Council of Governments, Washington, DC.
MWCOG, 1994. A Blueprint for the Restoration of the Anacostia Watershed. Publication no. 94710. Metropolitan Washington Council of Governments, Washington, DC.
National Research Council, 1991. Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy. National Academy Press, Washington, DC.
Sharitz, R.R., and J.W. Gibbons (eds.), 1989. Freshwater Wetlands and Wildlife: Proceedings of a Symposium. CONF-8603101 (DE90005384). U.S. Department of Energy, Washington, DC. 1265pp.
USDA, 1985. Riparian Ecosystems and Their Management: Reconciling Conflicting Issues. GTR RM-120. U.S. Department of Agriculture, Forest Service, Fort Collins, Colorado.
|
NCSU Water Quality Group |
|