Riparian Restoration
Riparian buffers are critical vegetative areas directly adjacent to streams and rivers. These areas reduce/prevent bank erosion/ stream turbidity, contaminants from entering the stream, spread of invasive plant species, and stream water overheating. They also provide habitats for a range of species and offer a place for flood waters to dissipate before reaching urban infrastructure. Outreach and education of local community members is crucial for the restoration and implementation of riparian buffers, as many people view them as messy or as obstructions to views of the water and as a result mow their land to the very edge of the stream. This is especially concerning on agricultural land, as this increases the risk of excess fertilizer/pesticides entering streams from runoff in addition to the degradation of the overall stream structure (Kenwick et al., 2009).
The main riparian buffer functions are stream bank erosion control, pollutant absorbency, temperature control (from shade), and wildlife habitat are all critical for maintaining ideal habitat conditions. 15 meters is the minimal baseline to generate a basically functional riparian buffer, although 100 meters is the idea length to fully mitigate against polluted runoff and flood scenarios. Shade provided by the canopies of large trees is critical for the survival of native trout, which require cold water habitats. Areas lacking trees along the stream bank are at risk of overheating on hot, sunny summer days. The trees’ roots are also capable of stabilizing the banks to prevent erosion and slow the rate of runoff enter the stream. Urban streams increase their velocities exponentially during storm events due to the rate of runoff entering them. This creates a deeper center channel, disrupting the naturally rocky, shallow stream beds needed for native aquatic species. Riparian buffers also naturally possess limited phytoremediation abilities, as trees, shrubs and grasses are adapted to intercept 90- 95% of sediments and remove the phosphorous attached to it (up to 85%). Sedge, switchgrass, and gamagrass have been proven to remove approximately 70% of petroleum hydrocarbons (TPH) after one year of growth.
Bioretention
Bioretention is the use of natural basins to collect stormwater runoff and any sediments/contaminants which may be included in the water, reduces the stress placed on stormwater infrastructure, and decreases the risk of flash floods. These may be as simple as grassy berms and swales, but more advanced and visually appealing bioretention areas have a range of plant species that can also create viable habitats, including constructed wetlands (Davis & Hsieh, 2003).
Sloped Marsh System
Retention basins and wet ponds are common stormwater control methods deployed across the landscape, and while effective at slowing and managing runoff rates, they tend to be lacking in terms of removing pollutants, especially ones dissolved within the water itself. To remedy this, extensive research has been placed into stormwater wetlands, which according to the EPA, “detain stormwater, remove pollutants, and provide habitat and aesthetic benefits.” These systems combine the retention abilities of traditional wet ponds with the natural phytoremediation capabilities of existing wetlands. However, stormwater wetlands are fundamentally different from natural wetlands in that their specific purpose is for stormwater runoff treatment, they have less biodiversity, and are heavily designed. It is not recommended to divert stormwater to natural wetlands despite their natural phytoremediation abilities because the intricate balance natural systems can be easily disrupted by sudden changes in hydrology and water quality. Stormwater wetlands require sufficient drainage areas (at least 10 – 25 acres as a minimum), dry weather base flows, a permanent pool level, and a proper maintenance plan. There are a range of design option for stormwater wetlands based on local conditions/landscape and desired effect, but the most common two are the pond system and the marsh system.
Floodplain Pond System
Each design possesses three main components: a permanent pool, a high marsh, and a low marsh. The pool zone consists of standing water 2-6 feet deep with submerged or floating vegetation. A forebay may be included by the inlet and a micropool by the outlet to help prevent clogging. The marsh zone is further divided into the high marsh (standing water of no more than 6 inches) and low marsh (standing water 6-18 inches). These areas offer emergent wetland vegetation and habitat for invertebrates, insects, and birds. Pond systems are focused on storing a large volume of water, with most of the area dedicated to the pool which is rimmed by low and high marsh. These systems are ideal for flood mitigation and require less area due to their deeper volume. Marsh systems are the inverse of pool systems, with most of the area devoted to low and high marsh which is supported by only a small micropool. Complex micro-topography allows for a higher diversity of plants capable of remediating any pollutants present in the water and also decreases the velocity of the runoff in these systems. A detailed plan is needed for both systems, highlighting the plant species to be used and their location in the stormwater wetland.
Phytoremediation
Phytoremediation is the implementation of plants to stabilize or reduce contamination of soils and water. It is a natural process that is favorable in conditions with shallow streams and low concentrations of contaminants. Specific plant species will be chosen based on the possible chemicals from fracking fluid and other pollutants related to agriculture and Marcellus Shale gas drilling. Factors that will impact the functionality of phytoremediation areas are regional climate, soil compatibility, ease of planting and maintenance, and ability to filter water (Comino et al., 2012). It will also be important to use native plants when possible. These systems will typically be found within bioretention areas and are a cost effective/less invasive method compared to conventional contaminant removal strategies.
Aquatic & Terrain
The U.S. Environmental Protection Agency defines phytoremediation as “the direct use of green plants and their associated microorganisms to stabilize or reduce contamination in soils, sludges, sediments, surface water, or ground water.” There are a range of types of phytoremediation, determined by where contaminants are intercepted, where they are transported to, and the final composition of former contaminants. The primary focus for all forms of phytoremediation is to reduce, remove, degrade, or immobilize contaminants. Phytostabilization includes the sequestration or immobilization of contaminants when they are either absorbed by the plant or simply interact with biochemicals exuded by the roots. Phytohydraulics utilizes deep rooted plants such as trees to contain, degrade, or sequester contaminants that are present in groundwater via contact with roots. Phytovolatilization does not store or degrade volatile contaminants. Instead they are released (volatilized) into the air through the plants’ stomata. Rhizosphere degradation functions similarly to phytodegradation, except that the degradation occurs due to the enzymatic microbial activity found around the plant roots rather than any processes occurring in the plant itself.
Floating Treatment Wetland
Phytoextraction primarily involves contaminants becoming absorbed by either the roots or root surface where they are taken up into the plant and accumulated within other plant tissues and cells. Proper disposal of plants used in phytoremediation is critical. While there are many regulations that must be adhered to, there are also a range of benefits able to be gained from this process. Biomass collected from phytoremediation can be placed into a lead or zinc smelter and fired, releasing stored metals in the form of oxides, reducing the dry weight of the biomass by 90%, and allowing for the re-collection of stored, valuable metals in the form of high grade ore. The burning process also generates electricity, which can be captured and used (1.2 kWh in one study). Floating treatment wetlands (FTWs) are floating rafts that grow plants that treat runoff pollutants. These are low to moderately low cost systems that can either be homemade (approximately $1 per sq. ft.) or purchased from a commercial retailer (approximately $24 per sq. ft.) Because contaminates are accumulated within the plants’ biomass, they must be harvested annually. September to October is the preferred time for full plant harvest, and July to August is preferred for aerial biomass harvesting only. Nutrients such as Nitrogen and Phosphorous from agricultural runoff are the most readily absorbed by plants used in FTWs, although there has been success in studies involving copper, zinc and other metals.
Large Scale Floating Treatment Wetlands in Tioga Lake
FTWs are also capable of offering habitat for fish, roosting places for birds, and shade to reduce overall water temperature. They can be placed in existing bioretention systems as a retrofit or on a larger scale such as walkable floating wetlands that can be added to lakes and offer a place to fish or launch small boats. Plant species that have been found to remediate water quality exceptionally well are pickerelweed (Pontederia cordata), soft-stem bulrush (Schoenoplectus tabernaemontani), tussock sage (Carex stricta), big blue stem (Andropogon gerardi), and marsh hibiscus (Hibiscus moscheutos). It is important to recognize that non-native species are occasionally superior in their ability to survive and accumulate toxic contaminate in comparison to native plants. If used, these non-natives must be monitored to ensure they do not spread unwantedly, however since these are not invasive species the risk of these species becoming more aggressive than the native cattail are low.
Small Scale Floating Treatment Wetlands in Bioretention System (Mansfield, PA)
This floating treatment wetland design (along the Mansfield Reservoir in Tioga County, Pennsylvania) represents a small scale proposal for an easily mobile and dynamic phytoremediative mitigation strategy for reducing surface water contamination from agricultural runoff and fracking spills. The design of the wetland structure provides natural habitat for fauna such as birds, butterflies, and fish (plant roots provide protection) and enhances the aesthetic beauty of the Pennsylvania landscape. In addition to using floating wetlands, riparian buffers are incorporated along the edges of the water body to minimize surface runoff, and bioretention measures (located on the right) to reduce risk of flash flooding. Overall, the design uses natural processes and cost-effective means to mitigate, regenerate, and protect these valuable ecosystems and water resources for future use.