Watershed CharacteristicsBulseco-McKim
University of Massachusetts Boston /Restoring Eelgrass to the Neponset River Estuary /
Watershed Characteristics /
Ashley Bulseco-McKim /
10/15/2012 /
ABSTRACT: This document synthesizes information gathered for the purpose of restoring eelgrass to the Neponset River Estuary. Phase one focuses on the physical and chemical aspects of the Neponset River Watershed, covering topics such as watershed area, basic river characteristics, land use types within the watershed, topography, surficial geology, bedrock lithology, atmospheric deposition, water supply and interbasin transfer, and aquatic habitat. /
PROBLEM STATEMENT
The Neponset River Watershed has a complex history of social, cultural, and economic development that has led to subsequent deterioration of its receiving estuary. The current goal of this semester’s class is to restore eelgrass to its estuary,[A1] in turn improving its overall health. Eelgrass performs a number of ecosystem services, including altered water flow, nutrient cycling, maintaining food web structure, providing a source of food and nursery habitat for upper trophic levels, stabilizing sediments, and contributing to the detritus pool. Unfortunately, multiple stressors, specifically sediment and nutrient run off, have caused massive seagrass decline (Orth et al. 2006).
The Neponset River Watershed Association (NepRWA) was originally formed in 1967 as the “Neponest Conservation Association” by a group of people who were concerned about the development and extension of Rt. 95 into Boston. They hold an everlasting commitment to an integrated, water-shed based approach that focuses on the protection and restoration of the physical, biological, and chemical integrity of the Neponset River. This report will help to answer a number of questions posed by the NepRWA, with the ultimate goal of restoring eelgrass and its corresponding ecosystem services to the Neponset River estuary.
In order to restore eelgrass to the Neponset River estuary, we must first grasp a number of fundamental characteristics regarding the watershed that feeds it. This report will focus on the physical and chemical aspects of the Neponset River Watershed, covering topics such as (in no particular order) watershed area, basic river characteristics, land use types within the watershed, topography, surficial geology, bedrock lithology, atmospheric deposition, water supply, and aquatic habitat.
WATERSHED DELINEATION
The Neponset River Watershed (total area = 340-km2)covers the western portion of the Boston Harbor watershed (Fig. 1), draining into the Neponset River and eventually reaching the coastal ocean at Boston Harbor (Zhu & Olsen 2009). The watershed itself consists of 14 cities and towns, including at least parts of Boston, Canton, Dedham, Dover, Foxborough, Medfield, Milton, Norwood, Randolph, Quincy, Sharon, Stoughton, Walpole, and Westwood (Fig. 2), and houses approximately 330,000 people (Neponset River Watershed Association – hereby NepRWA 2004). It is important to note that this calculation was likely performed using data from the 2000 U.S. Census, and will underestimate the current total population within the watershed. Current estimates may be more accurate using the 2010 U.S. Census. From the 1990 to the 2000 U.S. Census alone, population grew by around 6% in towns predominantly [A2]comprising the Neponset River Watershed, so it is clear that current population estimates should be used when considering the social aspects of this problem later in the semester.
The Neponset River Watershed can be further delineated into sub-watersheds or sub-regions (Fig. 3). Although they have not been given geographical names, they each correspond toa significant, nearby hydrological feature (e.g. Mill Brook; Table 1). Table 1 provides information on each sub-region, its total area in mi2[A3], and the percent area of forest, residential area, and “other” (NepRWA 1999). This information is useful because it provides a high resolution dataset of land cover, which is important in assessing overall impacts on watersheds and their receiving estuaries (see Land Use/Land Cover, pg. 3).
RIVER/STREAMS
The headwaters of the Neponset River originate in Foxborough at the Neponset Reservoir, a man-made impoundment (approximately 321 acres). After traveling 30 miles (48km) in a northeasterly direction, the river discharges into Dorchester Bay and Boston Harbor. The river faces impoundment by 12 dams, and passes through a number of private reservoirs before reaching its ultimate destination. There are a number of streams listed in Table 1 according to corresponding town and county, and Fig. 4, which provides a good visualization of the river’s branching patterns (NepRWA). For more details regarding river characteristics, please refer to Sarah Feinman’s assessment report.
LAND USE/LAND COVER [A4]
The Neponset River Watershed consists primarily of five major land cover types (as calculated from MassGIS data and ArcGIS 9.2 by Huang & Chen 2009): 38% of the watershed is residential[A5], 34% is forested (together they make up 72% of the entire watershed land cover), 5% is industry, 4% is wetlands, and 3% is golf courses (Huang & Chen 2009) (Fig.5). It is therefore safe to assume that there are relatively low levels of agriculture and horticulture in this area, and that the primary application of fertilizer will occur upon golf courses and residential lawns/parks.
These proportions of land use have undoubtedly shifted dramatically from pre-industrial times to modern day, showing a clear intensification of human alterations with population increase and urbanization (Bhaduri et al. 2000; Paul & Meyer 2008). Change in land use over timeis perhapsthe most significant human impact on hydrologic systems at the local, regional, and global scale (Bhaduri et al. 2000). Land use alterations have been shown to impact a number of bio- and chemical factors within the watershed, including Dissolved Organic Matter (DOM) quality and quantity (Wiegner & Seitzinger 2004; Huang & Chen 2009), average annual runoff (Bhaduri et al. 2000), species diversity (Minshall et al. 1985),and stream bed erosion (Schlosser 1991). In a study using the Little Eagle Creek watershed(Indianapolis, IN, USA) as a model, an 18% increase in urban or impervious areas led to an approximate 80% increase in annual average runoff and more than 50% in average annual loads for lead, copper, and zinc (Bhaduri et al. 2000). In light of such impacts, it is crucial that we continue to assess land use changes in the Neponset River Watershed (specifically a transition from forested/wetlands to urbanized/impervious areas) and their corresponding ecological consequences. Therefore, we may gain the ability to identify environmentally sensitive areas throughout the watershed, while evaluating alternative land use scenarios to reduce annual runoff and non-point source (NPS) pollution. Considering such a large proportion of the US population lives in metropolitan areas, it is inevitable that urban and industrial areas will continue to expand and human activity will continue to alter the environment (Clark 1967; Chinitz 1991). Understanding the impacts of land use change over a spatial and temporal scale will allow for prediction of environmental consequences and the development potential mitigation strategies.
One major consequence of urban development is the increased percentage of impervious surfaces, which can include roads, rooftops, sidewalks, patios, parking lots, and buildings, or any other material that prevents the infiltration of water into the soil (Arnold et al. 1996). These surfaces contribute to the environmental impacts of urbanization by inhibiting the natural process of groundwater recharge, thereby lowering water tables, and resulting in intermittent or dry stream beds during low flow conditions(Harbor 1994). Without the ability to infiltrate the soil, the water instead flows at a greater velocity and volume on the surface,resulting in large sediment deposits and aggravated downstream erosion (Arnold et al. 1996). Additionally, runoff contains a wide array of pollutants, including nutrients, pesticides, pathogens, oil, grease, sediment, and heavy metals, which accumulates from these same impervious surfaces, and eventually feeds into its receiving estuary (Arnold et al. 1996; Bhaduri et al. 2000).
Approximately 24% of the Neponset River Watershed’s total acreage consists of impervious surfaces (NepWRA 2004b; Fig. 6); however, the degree of imperviousness inherently differs between and within cities/towns. Much of this variation depends on the total amount of development, and the distribution of land use throughout the watershed.Based on work by the Massachusetts office of Coastal Zone Management (MCZM), the NepRWA described the degree of imperviousness according to land use for the Neponset River Watershed[A6]as: 54% imperviousness for Residential Less than ¼ acre, 30% imperviousness for Residential Multi-family ¼ to ½ acre, 30% imperviousness for Residential Multi-family over ½ acre, 58% imperviousness for Commercial/Industrial land use, and 51% imperviousness for Transportation (NepRWA 2004b). Should it then assumed that forest accounts for the remaining percentage of land cover in residential areas? [A7]
By understanding how impervious surfaces are distributed throughout the Neponset River Watershed, we may gain a better understanding of the degree to which they are impacting the receiving estuary. Studies have found a strong correlation between a drainage basin’s imperviousness and the physical/chemical health of its receiving stream (Klein 1979; Griffin et al. 1980; Schueler 1992), and according to information from the Center for Watershed Protection, expected watershed impacts worsen as percent imperviousness increases. At 0%-10% imperviousness, channels are stable, and support high water quality and excellent biodiversity. At 11-25% imperviousness, there may be some signs of degradation, some channel erosion and widening, some elevated nutrients and pathogens, and fair to good biodiversity. Lastly, at 26% or more imperviousness, there may be streambank erosion, channel instability, high nutrient levels, low biodiversity, and limited human contact with water supply due to high bacteria levels (NepRWA 2004b). Therefore, we must continue to consistently record percent imperviousness and changes in land use within the watershed as urban development continues at high enough resolution to allow for the continued analysis of watershed impacts.[A8]
TOPOGRAPHY/LITHOLOGY
The topography, surficial geology, and bedrock lithology can also play a role in watershed characteristics by influencing watershed boundaries, and waterinfiltration, flow, and chemistry, respectively (Fetter 2001). The topography of the Neponset River Watershed is fairly mild, with the steepest topography occurring at Buck Hill (Quincy; 496 ft), Chickatawbut Hill (Milton; 470 ft), Houghton Hill (Milton; 420 ft), Tucker Hill (Quincy; 499 ft), and Wolcott Hill (Milton; 470 ft) ( note, site may not be reputable[A9]). The Neponset River Watershed is delineated using these topographical features, and is defined separately from Boston Harbor Watershed’s other sub-watersheds (Fig. 1).
Precipitation that falls onto a watershed may travel as surface runoff, shallow interflow, flow through surficial features, or groundwater through bedrock fractures (Newton et al. 1987). Surficial geology of the Neponset River Watershed consists primarily of sand and gravel, large sand deposits, and to a lower extent, floodplain alluvium (OLIVER MassGIS). Sand and gravel ranges from 20-35% porosity, or the percent of void spaces between solid fragments, and between 10-2 to 103 intrinsic permeability (in darcys), representing the degree of ease with which a porous medium can transmit a liquid under a potential gradient. It is also tends to have a higher hydraulic conductivity, which is a coefficient that helps to define the rate at which water can move through a permeable medium (Fetter 2001). These characteristics suggest that water will be capable of infiltrating the surface soil in the absence of impervious surfaces; although, more research should be conducted on the available literature to gain a better understanding of the interaction between surficial geology and water flow. This could have tremendous implications on the vertical and horizontal movement of groundwater, as well as velocity, which will eventually translate to groundwater discharge impacts to the Neponset River Watershed streams and estuary. [A10]
Bedrock lithology, or mineral composition/classification of rocks, can also play a role in defining water flow and water chemistry. The Neponset River Watershed is comprised of primarily granite (which groundwater cannot infiltrate), basin sedimentary rock, and metamorphic rock near the coastline. The extent of fractures within the bedrock and the degree of porosity can ultimately influence the residence time of the water underground, while the mineral composition can lead to a number of chemical transformations of the groundwater itself. These characteristics can further be applied to the analysis of water quality in both surface water and shallow groundwater, along with the investigation of stream sediments in regards to bedrock lithogeochemistry (Fetter 2001). Additionally, rare earth elements (REE) have been used to study a variety of geological processes regarding chemical weathering and water-rock interactions (Hannigan 2005). Again, more research needs to be conducted on the existing literature to understand the complex interaction between bedrock lithology and water features to further characterize the essential information required to restore eelgrass to the Neponset River estuary.
WATER SUPPLY AND INTERBASIN TRANSFER
Approximately 220,000 people use water that at least partly depends on groundwater from the Neponset Valley (NewRWA). Of that pumped water, 21% is pumped back into the Neponset River Watershed as septic system effluent, and 65% is relocated outside the basin via sewer systems (NepRWA 1998).Along the way, water is lost to a number of sources; however, aging sewage infrastructure exacerbates the extent to which groundwater is lost by leaking and infiltration (NepRWA 2004b). Due to these losses, the Neponset experiences an annual net loss of 9 billion gallons water per year, a volume approaching ¼ of the river’s annual discharge.
The NepRWA attributes this extensive net loss to a number of factors: 1. The extension of sewer lines for service of both new and existing development (formerly septic), 2. The municipal development of new water supply sources throughout the Neponset River Watershed to sustain increasing water demand due to population rise or increased per capita demand, 3. The development of new Neponset water supply sources instead of MWRA (due to a rise in cost and compliance with the Federal Safe Drinking Water Act), and 4. The installation private irrigation wells in attempts to avoid compliance with water use restrictions opposed during periods of drought (NepRWA 2004b). [A11]
Table 4 presents current Neponset Municipal Water Supply Sources and Wastewater Infrastructure according to town as either ‘Neponset’, ‘MWRA’, or ‘other’. It is interesting to see the different trends in water supply dependence (e.g. Canton relies heavily on the MWRA while Sharon relies more so on their local supply). An important social aspect of this project to be researched further is the individual attitudes and rationale from each town, [A12]leading to patterns in water usage and dependence. There is also a knowledge gap (especially in my own knowledge) in wastewater infrastructure and its current status (e.g. pipe integrity and age) throughout our watershed per municipal. We need to determine the distribution of sewage or septic systems (perhaps in light of Title V to consider social aspects), evaluate the extent of failing sewage systems, and further research processes of inflow and infiltration to the best of our ability, although a number of assumptions will have to be made due to the lack of availability of this type of information.These steps will help us to determine rates and distribution of sewage disposal, calculate inputs lost to sewage and septic systems (e.g. nitrogen loads), and investigate a number of other environmental impacts of NPS[A13] pollution.
ATMOSPHERIC DEPOSITION
Atmospheric deposition is the transfer of pollutants from the air to the earth’s surface (EPA 2012), and can occur in two methods: wet deposition and dry deposition. Wet deposition is when aerosols or other gases are dissolved/suspended in precipitation (e.g. rain, snow, sleet). [A14]This can typically be measured directly by analyzing trace quantities of pollutants via Inductively Coupled Plasma Mass Spectrometry (ICP-MS), or other analytical instrumentation, and multiplying the resulting concentration by the total volume of precipitation over a specified time period. Dry deposition, on the other hand, consists of suspended particles or gaseous contaminants which gravitationally settle onto land or water surfaces. Once deposited onto the earth’s surface, the contaminants quickly adsorb to vegetation, soils, or any other surfaces present in that given area (Zhu & Olsen 2009). As suggested by a smaller volume of literature, dry deposition is more difficult to directly measure. Generally, investigators will collect particles using an artificial surface that represents a naturally occurring surface, or will take the concentration of contaminants present in the air and multiply that by the values of deposition velocities as found in the current literature (Golomb 1999).
In the Valiela et al. 1996 article [A15]we covered in class, authors created and implemented a model to estimate nitrogen loading from a coastal watershed to its receiving estuary. Nitrogen loading to the coastal waters is of major concern, particularly because primary production is limited by nitrogen availability (Valiela 1995; Howarth 1988). As a result, it is not only important to enumerate nitrogen loading from fertilizer and wastewater disposal, but it is also important to know how much of that nitrogen is being deposited in residential areas (impervious surfaces), forests, or various other surfaces via wet and dry atmospheric deposition.In the WBLMER model, atmospheric deposition was estimated to be twice that wet deposition (in kg N ha-1 yr-1) due to the lack of studies covering dry deposition. A handful of studies have investigated the rate of dry deposition in the Boston area (e.g. Zhu & Olsen 2009 looked at 7Be deposition from the roof of UMass Boston; Golomb 1999 looked at toxic metals Cd, Co, Al, Cr, Cu, Fe, Mn, Ni, Ph, Zn, Hgbiweekly in Massachusetts Bay); however, I suggest a number of atmospheric deposition studies be conducted directly in our estuary. Because a large portion of the Neponset “airshed” is urban, to what extent will dry deposition contribute to total atmospheric deposition as a whole? [A16]
It is clear that atmospheric deposition plays an important role in loading of various trace metals and pollutants to the coastal ocean (Valiela et al. 1996); however, it must be readily monitored over long time scales in order to capture its spatial and temporal variability. Rates of wet deposition depend on precipitation and its particular chemistry, while rates of dry deposition depend on wind velocity and direction. Therefore, an understanding of climate is required to fully interpret effects of atmospheric deposition – climate has not been fully discussed in this assessment report, but Table 3 provides the precipitation rates (inches per month) for East Boston and Milton (representing the NE Region) and Foxborough and Milton (representing the SE Region). More detailed climate characteristics, including information regarding wind, should be further researched. USGS or NOAA NCDC will likely report reliable data from which we can infer climate and atmospheric deposition rates.