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.

ABSTRACT


Camp 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.

 Coring 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.

Nested 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

 Water-level 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. 

The 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.

 The 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.

 The 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.