WATER LEVEL MONITORING OF AN EXISTING WETLAND AND A RESTORED WETLAND AT CAMP DODGE 1996-1999: A SUMMARY REVIEW
R.D. Rowden, C.A. Thompson, M.C. Jones
Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 43, 2000, 64 p.
Dodge is a 4,400-acre National Guard training facility located in central Iowa
on the southern terminus of the Wisconsinan Des Moines Lobe (DML) in an area
previously containing numerous prairie pothole and riparian wetlands.
The advance of the DML resulted in the development of a complex of linked
depressions, which may act as a relatively permeable, shallow subsurface
drainage system. Land acquisitions
by the facility afforded the opportunity to conduct an integrated wetland
restoration with the emphasis of the study on restoring the sedge edge
associated with prairie pothole wetlands. In 1995, researchers from several universities and
state and federal agencies began studying the hydrology, water quality and flora
and fauna of an existing prairie pothole and a previously drained wetland
located within the facility. The
existing wetland is located in an upland, semi-closed depression and the
restoration site is located 0.5-mile northwest in an abandoned glacial outwash
channel. The restoration site had
been artificially drained by a tile system, which was cut at several locations
in June 1996. Monitoring wells were
installed at both sites, and the existing wetland served as a reference site for
the restoration. This report
summarizes the results of water-level monitoring at the two sites during water
years (WYs) 1996-1999.
was done to better understand the geomorphology and history of the sites.
The restored site was dated to 9,970 ± 70 RCYBP and appears to have been
active until recent times. No
hiatuses were visible in the cores, although water level declines would have
been expected in the middle Holocene, which was warm and dry in the study area
and may have led to peat degradation. The
stratigraphy is relatively continuous over the restoration site and from top to
bottom consists of a dry to wet muck, a highly compressed fibric carbonate-rich
peat, muck to peat with abundant shells, silty clay, highly organic muck grading
to gyttja, all overlying outwash sands and gravel.
The silt layer may correspond to the warm and dry period of the middle
Holocene, however further dating is needed to better explain the observed
sequence. The stratigraphy of the
pothole from top to bottom consists of Holocene-age muck, till-derived colluvium,
and then peat overlying late Wisconsinan glacio-fluvial sand.
The peat was dated to 11,540 ± 60 RCYBP.
wells within both wetlands were instrumented with data-loggers and pressure
transducers to continuously monitor changes in hydrology as the restoration
progressed. Water levels at the
restoration site remained below the land surface through WY 1996 and early WY
1997, then following a wet late summer-early fall, water levels rose and have
remained above the land surface since February 1997. At the restoration site, surface water levels have been
essentially coincident with groundwater levels, suggesting a greater reliance on
groundwater inputs. This is also
indicated by few sharp increases in well water levels corresponding to
precipitation events. The upland well has shown the most variation, and
indicates that during most of the year groundwater is generally moving into the
wetland. In the fall, during drier
conditions, groundwater movement has been from the wetland into bank storage
data for the existing wetland showed less variability than the restored site
during WY 1996 and early WY 1997, but greater short-term variability than the
restored site during WYs 1997-1999. This
is consistent with the ephemeral nature of the pothole.
Sharp responses of water levels within the wetland corresponding to
precipitation events suggest a greater dependence on precipitation and/or
overland flow than groundwater inflows. A
water budget previously developed corroborates this and shows that precipitation
supplies in excess of 80% of the overall wetland budget, followed by groundwater
inflow and runoff. Groundwater
levels were often below the land surface during late summer and early fall when
surface water was normally 2-3 feet deep in the wetland indicating net seepage
out of the wetland. Well levels in both wetlands showed that local groundwater
flow is generally directed from the northwest to the southeast.
Water level data from upland wells near the study area also suggest that
the groundwater flow in the area is from northwest to southeast.
increasing similarity of longer-term water level responses of the upland wells
in the restoration and existing sites suggests that hydrologic conditions at the
restoration site are approaching a more natural balance.
The more stable water levels within the restoration site during the
latter half of the monitoring period suggest that the restoration site may be
less ephemeral than the existing site.
water quality of both wetlands was similar to historical water quality data and
is representative of groundwater flowing through unconsolidated materials in
this area of the DML. The
concentrations of most ions were greater in the groundwater than in the surface
water of the wetlands and generally increased toward the end of summer due to
evaporative effects and decreased in the fall due to dilution from increased
precipitation. The concentrations
of pesticides, nitrates and phosphates were relatively low in both wetlands,
probably due to the effects of dilution, volatilization and/or consumption by
microorganisms within the wetlands.
overall flow direction of groundwater in both wetlands appears to parallel
Beaver Creek and may support the hypothesis of a linked-depression system.
Horizontal gradients suggest that groundwater may be flowing through a
shallow sand channel approximately 2-3 feet below the land surface.
There may also be deeper sand channels located below the bottom of the
wetlands connecting the wetlands to the larger Beaver Creek drainage system.
Further study might employ a combination of ground-penetrating radar and
a geographic information system to map shallow sub-surface sand bodies.
Drilling transects across some of the sand bodies would confirm their
structure. Data currently being
collected by the Iowa Department of Natural Resources Geological Survey Bureau
from monitoring wells located throughout Camp Dodge will be used to create a
regional water table map, which should assist in future investigations.