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The following article was written by Russell Urban-Mead, and presented to The Great Swamp Watershed Conference on October 4, 1997
Russell Urban-Mead, M.S.
Hydrogeologist
The Chazen Companies
The Great Swamp watershed extends from the Town of Southeast in Putnam County to the Town of Dover in Dutchess County. The watershed commands significant portions of the intervening Towns of Patterson and Pawling and embraces a total of approximately 63,000 acres or 98 square miles. Two principal rivers drain precipitation falling on this extensive region. These are the East Branch Croton River which flows southward from Pawling toward Southeast, and the Swamp River which flows northward from Pawling into Dover. The East Branch Croton River and the Swamp River share joint headwaters in the Great Swamp wetlands near the Village of Pawling where an indistinct groundwater divide separates northflowing water from southflowing water.
The two rivers occupy a deep and broad erosional valley worn into the otherwise rugged upland watershed terrain of resistant metamorphic gneisses, schists, and other ancient crystalline rocks. The erosional valley was worn into an exposed band of softer carbonate rock consisting of dolomitic marble, semi-pure marble, and carbonate sandstones, which yielded slowly to millions of years of river erosion and intermittent periods of glacial scour. In most areas, the river valley walls rise abruptly from a +/- 400 foot elevation in the Great Swamp lowlands to over 700 feet in the adjacent watershed uplands.
Recent glacial activity left the river valley filled with an untidy plain of mixed sediments up to ± 30 feet deep. Some of these sediments were deposited when the glacier was still nearby and cascading meltwater could transport great masses of coarse sand and gravel seaward. But as the glacier receded toward Canada, the icemelt flows chose other routes, leaving the Great Swamp valley to stagnate and fill its passageways with residual silt and clay. Today, the East Branch Croton River and the Swamp River are mere vestiges of the short-lived but vigorous glacial outwash torrents which once occupied this valley. The former floodways and silt-filled lowlands are now exposed wetlands, nearly coincident with the elevations of the adjacent rivers but seldom fully inundated except during exceptional flood events.
Precipitation falling on the vast Great Swamp watershed flows toward the lowest parts of the watershed in an inexorable search for the sea. Runoff enters gullies and streams to flow toward the two rivers of the Great Swamp. Rainfall also attempts shortcuts by infiltrating down into fractures and other openings in the watershed's varied geologic formations. Once the fractures and openings become filled with water, the below-ground flows ("groundwater") migrate slowly toward the Great Swamp drainage; to reemerge in the wetland areas and rejoin the surface water flows to the ocean. Thus, groundwater is not a miraculous source of "new" water; rather, it is merely precipitation which has unsuccessfully attempted a short-cut to the sea.
The 4,000 wetland acres of the Great Swamp are therefore, in general, receiving areas for groundwater, and the Great Swamp should not normally be thought of as an aquifer recharge area. (In fact, very few wetlands are ever perennial recharge areas. Wetlands remain wet during droughts precisely because they are sustained by discharges of groundwater at times when upland areas are drought-striken.) But this is not to say that groundwater protection or planning measures are not necessary in the valley occupied by the Great Swamp. The glacial sediments in the valley bottom and the underlying carbonate geologic formation offer good opportunities for well drilling and often pumping of these wells reverses the normal discharge of groundwater. Under these pumping conditions, it is possible for contaminants to be drawn down into the Great Swamp aquifer system. During heavy flooding, high water in the swamp can also temporarily reverse the normal upwelling of groundwater, forcing brief but potentially deep-penetrating, movement of surface water into the underlying aquifer.
The aquifer system under the Great Swamp is a unique hydrogeologic entity because the carbonate formation is somewhat more fractured than any of the surrounding upland geologic formations. The carbonate is also often in direct contact with overlying saturated glacial sediments. The fractures and glacial sediments together offer highly permeable pathways for groundwater flow and wells in the carbonate formation generally offer, higher flows than do wells drilled in upland areas. The fractures and sediment pore spaces also provide extensive storage capacity for groundwater. Thus wells drilled in the Great Swamp valley bottom continue to yield water long after wells drilled into less extensive upland fracture systems have run dry.
The Great Swamp valley bottom aquifer system is hydraulically well-suited to providing reliable flows of well water because of an unusual concentration of water-bearing fractures and pore-spaces, and also because the valley bottom aquifer receives the groundwater discharge from the entire watershed. Using a rough estimate of 7 inches of groundwater infiltrating the entire watershed each year, approximately 12 billion gallons of groundwater move through the watershed each year. A significant portion of this discharge provides the essential river baseflow which sustains the wetlands and rivers during dry periods.
This baseflow should not to be confused with an even greater volume of surface runoff which flows through the Great Swamp each year. Some portion of the discharging groundwater is required to sustain the ecological requirements of the Great Swamp wetland and the New York City reservoirs to the south, but a portion of this flow is also available to humans living in the watershed. Indeed, a significant quantity of this water is already used by the existing human population. Some of the best locations for extracting this critical water resource are found in the Great Swamp valley bottom.
Groundwater sustaining the Great Swamp valley bottom normally offers excess capacity available for human uses. However, several threats to the sustainabiity of the groundwater are worth mentioning:
1. The Quantity of available water is entirely dependent upon precipitation cycles. During drought periods, up to 30% less, or only approximately 8 billion gallons of groundwater may be discharging annually as baseflow to the Great Swamp rivers. We must learn how much baseflow is required by the ecological systems of the Great Swamp. This will allow us to determine how much additional water is available for human consumption. (The human taking of water can be multiplied if human wastewater is re-contributed to the groundwater flow equation in a condition suitable for reuse.),
2. The Quality of water available from the Great Swamp aquifer system is dependent upon wise land use planning in the valley bottom and the upland watershed areas. Contaminant releases in the valley bottom can threaten human water supplies where wells draw surface water into the aquifer, or whenever stormwater, surges drive contamination into .the aquifer. Upland accidental discharges or chronic non-point source contaminants also migrate through fracture pathways toward the Great Swamp, threatening water quality in all upland wells along the groundwater flow path as well as in valley-bottom water wells.
Contaminants introduced in upland areas often benefit from decay occurring during the slow migration of the groundwater and from dilution by additional precipitation; nonetheless, all upland pollution sources threaten the quality of the water resource in the Great Swamp. Common sources of contamination include human/animal waste (e.g. nitrogen or bacteria), synthetic chemicals (e. g. pesticides, solvents, petroleum products), and inorganics (e.g. road salt or water softener salts).
The human population clearly benefits from the low-cost and high-quality of the Great Swamp water resources. To sustain this benefit, we need to .preserve the Quantity and Quality of this resource.
1. To preserve Quantity, we must institute programs which track the water budget of the watershed and monitor, at some scale, the groundwater takings and returns (wastewater). This effort will identify whether our taking is reasonable, whether additional stormwater infiltration programs are necessary to recharge the groundwater budget, and will allow earlier responses to droughts as they occur. We should also assess the ecological requirements of the Great Swamp to ensure that human takings do not exceed the wetland's requirements. Baseflow measurements, streamflow measurements, and aquifer waterlevel data are all critical data.
2. To preserve Quality, we must be supportive of existing programs and regulations which limit threats to water quality. We should also identify and close existing program or Agency "gaps" which may allow continuing risks of contamination. We must also track human/animal wastewater returns to ensure that nitrogen and bacteria levels in groundwater do not rise above acceptable levels; where septic wastes are returned to ground, these must be balanced by suitable dilution by infiltrating precipitation. Finally, long-term monitoring of groundwater and surfacewater quality is necessary to provide baseline data identifying long-term quality trends. These data will potentially identify and then justify any required corrective actions in the watershed. The data collection effort might include routine sampling of an array of monitoring wells installed across the watershed, surface water sampling at appropriate locations, and coordinated ongoing evaluations of water quality data supplied by pubic water suppliers.
The Dutchess County Water & Wastewater Authority has recently completed formulating a water supply protection strategy. The plan involves a strategy for protecting valley bottom aquifer systems and offers valuable comment on planning options. A copy of the draft recommendations coming from this program is attached. Many of the Dutchess County concepts are directly translatable to the dual-County Great Swamp watershed.
Mr. Urban-Mead is a Senior Hydrogeologist working for The Chazen Companies In Poughkeepsie, NY, (914) 454-3980. Mr. Urban-Mead is currently completing an evaluation of the hydrogeology of the Town of Dover and has begun a comprehensive water resource assessment of the Harlem Valley basin. Both works involve assessments of the groundwater resource, identification of water resource threats and limitations, and development of recommendations for sustainable utilization and reasonable protection of valuable water resource.
The Chazen Companies September 15, 1997