Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York.
Part 4. Quantity and Quality of Ground-Water and Tributary Contributions to Stream Base Flow in Selected Main-Valley Reaches
by Paul M. Heisig
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5018
ABSTRACT
Estimates of the quantity and quality
of ground-water discharge from valley-fill deposits were calculated
for nine valley reaches within the Pepacton watershed in southeastern
New York in July and August of 2001. Streamflow and water
quality at the upstream and downstream end of each reach
and at intervening tributaries were measured under base-flow
conditions and used in mass-balance equations to determine
quantity and quality of ground-water discharge. These
measurements and estimates define the relative magnitudes
of upland (tributary inflow) and valley-fill (ground-water
discharge) contributions to the main-valley streams and
provide a basis for understanding the effects of hydrogeologic
setting on these contributions. Estimates of the water-quality
of ground-water discharge also provide an indication of the
effects of road salt, manure, and human wastewater from
villages on the water quality of streams that feed the Pepacton
Reservoir. The most common contaminant in ground-water
discharge was chloride from road salt; concentrations were
less than 15 mg/L.
Investigation of ground-water quality within a large
watershed by measurement of stream base-flow quantity and
quality followed by mass-balance calculations has benefits
and drawbacks in comparison to direct ground-water sampling
from wells. First, sampling streams is far less expensive than
siting, installing, and sampling a watershed-wide network of
wells. Second, base-flow samples represent composite samples
of ground-water discharge from the most active part of the
ground-water flow system across a drainage area, whereas
a well network would only be representative of discrete
points within local ground-water flow systems. Drawbacks
to this method include limited reach selection because of
unfavorable or unrepresentative hydrologic conditions,
potential errors associated with a large number of streamflow
and water-quality measurements, and limited ability to
estimate concentrations of nonconservative constituents such
as nutrients.
The total gain in streamflow from the upper end to the
lower end of each valley reach was positively correlated with
the annual-runoff volume calculated for the drainage area
of the reach. This correlation was not greatly affected by
the proportions of ground-water and tributary contributions,
except at two reaches that lost much of their tributary flow
after the July survey. In these reaches, the gain in total
streamflow showed a negative departure from this correlation.
Calculated ground-water discharge exceeded the total
tributary inflow in each valley reach in both surveys. Groundwater
discharge, as a percentage of streamflow gain, was
greatest among reaches in wide valleys (about 1,000-ft wide
valley floors) that contain permeable valley fill because
tributary flows were seasonally diminished or absent as
a result of streambed infiltration. Tributary inflows, as a
percentage of streamflow gain, were highest in reaches of
narrow valleys (200-500-ft wide valley floors) with little
valley fill and high annual runoff.
Stream-water and ground-water quality were
characterized by major-ion type as either (1) naturally
occurring water types, relatively unaffected by road salt, or (2)
road-salt-affected water types having elevated concentrations
of chloride and sodium. The naturally occurring waters were
typically the calcium-bicarbonate type, but some contained
magnesium and (or) sulfate as secondary ions. Magnesium
concentration in base flow is probably related to the amount of
till and its carbonate content, or to the amount of lime used on
cultivated fields within a drainage area. Sulfate was a defining
ion only in dilute waters (with short or unreactive flow paths)
with low concentrations of bicarbonate. Nearly all tributary
waters were classified as naturally occurring water types.
Ground-water discharge from nearly all valley
reaches that contain State or county highways had elevated
concentrations of chloride and sodium. The mean chloride
concentrations of ground-water discharge--from 8 to 13
milligrams per liter--did not exceed Federal or State
standards, but were about 5 times higher than naturally
occurring levels. Application of road salt along a valley
bottom probably affects only the shallow ground water in the
area between a road and a stream. The elevated concentrations
of chloride and sodium in the base-flow samples from such
reaches indicate that the concentrations in the affected ground
water were high enough to offset the low concentrations in all
unaffected ground water entering the reach.
Nutrient (nitrate and orthophosphate) concentrations
in base-flow samples collected throughout the valleyreach
network could not generally be used to estimate their
concentrations in ground-water discharge because these
constituents can be transformed or removed from water
through biological uptake, transformation, or by adsorption on
sediments. Base-flow samples from streams with upgradient
manure sources or villages served by septic systems
consistently had the highest concentrations of these nutrients.
Citation: Heisig P.M., 2004,
Hydrogeology and Water Quality of the Pepacton Reservoir Watershed in Southeastern New York.
Part 4. Quantity and Quality of Ground-Water and Tributary Contributions to Stream Base Flow in Selected Main-Valley Reaches:
U.S. Geological Survey Scientific Investigations Report 2004-5018, 21 p.
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