Water Quality Monitoring
USGS
Gauging Station at Greensboro, MD (01491000)
History | Hydrology & Density
of the Choptank | Photo
Archive
Only a small fraction
of the Choptank River basin is gauged (Fig. A5). The
USGS gauging station at Greensboro MD integrates the
hydrology of 18% of the Choptank basin (Lee et al 2000)
and was established in 1948 (link to USGS site).  As
part of an ongoing program which monitors water quality
in the Choptank River Basin, we have been regularly collecting
water samples since 1979 from the USGS gauging station
on the river near Greensboro, MD. Samples have been analyzed
for pH, conductivity, and concentrations of ammonium
(NH4), phosphate (PO4), nitrite + nitrate (NO2 + NO3),
total nitrogen (TN), and total phosphorus (TP). Analysis
of the first eleven years of these data is available
in Fisher et al (1998).
Greensboro
Sampling History (back
to top)
In October 1979 we began an intensive
monitoring program at the USGS gauging station. Samples
were collected manually by Robert and Edna Hanley,
who lived near the gauge site, and we picked up and
processed the samples monthly. This program continued
until 2000, when the Hanleys retired and relocated.
On May 31, 2000, in a joint program
with USGS, an ISCO 6700 autosampler was installed
in the USGS gauging station and the sampler now takes
daily samples composited on a weekly basis. In addition,
USGS uses a  modem
on the sampler to take discrete samples 15-25 times
per year.
Hydrology
and density of the Choptank River (back
to top)
Inter Annual variations in discharge
at Greensboro are caused by variations in rainfall
(Fig.
1). Discharge varies by a factor of 3-4 between
dry and wet years, and there are cycles of wet and
dry years with about a 5-10 year period. During 1949-2001,
there are no long term trends in discharge; however,
during the Bay Program's monitoring record, which
began during a dry year (1985), there is a significant
positive trend in discharge which is caused only
by the timing of the starting year.
2002 was unusually dry, especially
during the end of the summer. Very low rainfall and
persistently high temperature resulted in extremely
low flows that were ~10% of long term means for July
and August (Fig.
2).
N and P concentrations vary considerably
with stream flow. Figure
3 shows a time series of rainfall data in fall
1982 (black bars in top panel) which gradually saturated
soils, and the third storm on day 76 (Dec 15) caused
a major increase in stream flow (solid circles in
top panel). At the peak of the flow event, nitrate
and TN concentrations (middle panel) were depressed
by overland flow of rain water with relatively low
N diluting groundwater with high N, especially nitrate
content. In contrast, P concentrations (lower panel)
increased with flow. The small panels at the bottom
show the concentrations of N and P as a function
of discharge. The high concentration of nitrate in
groundwater are caused by fertilizer applications
and leaching from septic systems (Fig.
4).
There are significant trends in water
quality at the Choptank station. Nitrate, in particular,
has been increasing over the period of record (Fig.
5). In the 1960's, nitrite and nitrate concentrations
(dominated by nitrate, NO3) was 30-70 uM (0.42-0.98
mg NO3-N/L). In the late 1990's, nitrate concentrations
were 70-90 uM (0.98-1.26 mg NO3-N/L), about a 50
percent increase compared to 30 years earlier.
Waste
water treatment Plants (back
to top)
There are eleven waste water treatment
plants in the Choptank River basin licensed by Maryland
Department of the Environment (Table 1). Of these
eleven, the two largest, Cambridge and Easton, discharge
__% of the total waste water discharge. All of the
plants in Table 1 have secondary treatment (removal
of biosolids and biological oxygen demand, but most
of the original nutrients remain in solution), and
several have-or are adding-tertiary treatment to
remove N and P.
Sewage effluent samples
have been collected from the two largest Choptank
treatment plants, Cambridge and Easton, since 1984.
At that time, there was only annual analysis of effluent.
These plants discharge treated sewage into the river
and Councell Creek (see map). The wastewater samples
which we collect are processed for both TN and TP
measurements.
Atmospheric
deposition of C, N, and P |
