G. Richard Whittecar

Use of hydrogeomorphic concepts to design created wetlands in southeastern Virginia

Abstract Mitigation wetlands constructed in southeastern Virginia during the past decade have experienced problems with inappropriate water levels, excessive erosion and sedimentation, low levels of soil organic matter, overly compacted substrates, and sulfidic soils. Most of these problems can be recognized in the future with adequate planning that permits sufficient study of the geomorphic and hydrologic processes active at the mitigation site, and if greater attention is given to the history of geomorphic processes that created natural wetlands in that area. New procedures of assessing wetland functions that use the “hydrogeomorphic” (HGM) classification of wetlands require examination of both surface and subsurface processes. If these HGM concepts are expanded to include geomorphic evolution, they will greatly improve recent practices in the design and construction of mitigation wetlands.

Potential contaminants at a dredged spoil placement site, Charles City County, Virginia, as revealed by sequential extraction

Backfills of dredged sediments onto a former sand and gravel mine site in Charles City County, VA may have the potential to contaminate local groundwater. To evaluate the mobility of trace elements and to identify the potential contaminants from the dredged sediments, a sequential extraction scheme was used to partition trace elements associated with the sediments from the local aquifer and the dredged sediments into five fractions: exchangeable, acidic, reducible, oxidizable, and residual phases. Sequential extractions indicate that, for most of the trace elements examined, the residual phases account for the largest proportion of the total concentrations, and their total extractable fractions are mainly from reducible and oxidizable phases. Only Cd, Pb, and Zn have an appreciable extractable proportion from the acidic phase in the filled dredged sediments. Our groundwater monitoring data suggest that the dredged sediments are mainly subject to a decrease in pH and a series of oxidation reactions, when exposed to the atmosphere. Because the trace elements released by carbonate dissolution and the oxidation (e.g., organic matter degradation, iron sulfide and, ammonia oxidation) are subsequently immobilized by sorption to iron, manganese, and aluminum oxides, no potential contaminants to local groundwater are expected by addition of the dredged sediments to this site.

CONVERSION OF POTOMAC RIVER DREDGE SEDIMENTS TO PRODUCTIVE AGRICULTURAL SOILS 1

River channel and harbor dredging activities in the eastern USA generate hundreds of millions of yards of dredge materials annually with very little used beneficially. The Woodrow Wilson Bridge project across the Potomac River at Washington D.C. generated in excess of 450,000 m 3 of silt loam, high pH, low salt dredge materials. The materials were barged to Shirley Plantation on the James River in Charles City Co. Virginia, and placed into an upland utilization area atop a previously reclaimed sand and gravel mine. The strongly reduced inbound sediments were very low in sulfides, pesticides, and other contaminants. The materials were dewatered, treated with varying rates of yardwaste compost and planted to wheat (Triticum vulgare) in the fall of 2001 and corn (Zea mays) in 2002 and 2003. Winter wheat yields in 2001 were similar to local agricultural lands despite animal damage and less than ideal establishment conditions. Average corn yields in 2002 were greater than long-term county prime farmland yields in a severe drought year (2002) and equaled county averages in a wet year (2003). Soil pit and auger observations revealed significant oxidation and formation of a deep Ap-AC-C profiles with coarse prismatic structure within two years after placement. Overall, the chemical and physical properties of these materials are equal or superior to the best topsoils in the region, supporting federal initiatives to utilize suitable dredge materials in upland environments whenever possible.

Sedimentology and palynology of Middle Wisconsinan deposits in the Pecatonica River valley, Wisconsin and Illinois

Abstract The bedrock valley of the Pecatonica River north of Freeport, Illinois, contains a thick valley-fill complex of alluvium and drift. Within the valley, loess-capped benches surround hills of silty Illinoian drift. Beneath these benches lie thick deposits of poorly sorted stony silt interbedded with thin lenses of silt, sand, and organic-rich loam. Channel deposits and peat cap the diamicton in places. We interpret the stony silts as solifluction debris shed from silty slopes within the valley-fill during the Early or Middle Wisconsinan (Altonian). Top and bottom radiocarbon dates from a 2.5-m section of peat overlying the diamicton are 26,820 ± 200 and 40,500 ± 1700 yr B.P., respectively. We informally refer to the stony silts, channel sediments, and peat as the “Martintown unit.” Geomorphic position, sediment input, and macrofossils suggest that the dated peat was deposited in a floodplain pond (oxbow?). The pollen record from the peat indicates that a boreal forest dominated this area during the Middle Wisconsinan (late Altonian and Farmdalian). Two pollen zones are recognized: a basal Zone I with Pinus slightly more abundant than Picea and with few herbs and shrubs, and an upper Zone II dominated by Picea and with a larger representation of herbaceous and shrub taxa. Little displacement of vegetation zones is indicated, even though ice advanced to within 100 km of the site during the time of peat accumulation. Because of the problems involved in clearly defining Middle Wisconsinan forest-tundra in mid-latitudes by using analogs of Holocene forest-tundra in high latitudes, caution is required in making geomorphic inferences solely from vegetation data. Together, though, pollen and sediment data indicate that during the Middle Wisconsinan, Pecatonica hillslopes progressed through a sequence of instability-stability-instability related to climatic fluctuations.

Estimating the yield of crushable stone in an alluvial fan deposit by electrical resistivity methods near Stuarts Draft, Virginia

Abstract A late Cenozoic alluvial fan complex near Stuarts Draft, Virginia, consists of three cobble and gravel units with different degrees of weathering. The youngest unit contains virtually unweathered quartzite clasts; the older two units, also derived from quartzite bedrock, range from being somewhat weathered with moderately competent clasts to very strongly weathered with cobbles that are incompetent and porous. Differences in porosity caused by these contrasts in weathering permit successful use of electrical resistivity sounding and profiling in and around an active alluvium quarry. With these methods we delineated the borders and bottom of the youngest, high resistivity gravel unit. An isopach map of this unit, based on the resistivity interpretations, indicates two paleovalleys where the minable unit is thickest. In the surveyed area, the paleovalley deposits are as much as 10 and 18 m thick and taper to 1–2 m toward the south. The weighted mean thickness of the gravel unit is 3.95 m, and the estimated volume of this unit, based on an isopach map, is about 5 × 106 m3.

Alluvial lithofacies recognition in a humid-tropical setting

Abstract Cobble gravel deposits in the Antigua Formation accumulated on a large alluvial fan or braid-plain west of the Cordillera Occidental in southwest Colombia. This formation was probably deposited during the Pleistocene in a very wet tropical climate (> 500 cm/yr rainfall). Fining-upwards sequences of clast-supported, imbricated boulders and cobbles dominate with maximum clast sizes between 30 and 300 cm. The sand matrix in the Antigua gravels and the minor (⩽ 10%) sand facies are weathered to clay at depths of up to 20 m. The sand facies contains abundant drift logs and leaf mats. Except for the absence of debris flows and the very coarse nature of the gravel, the Antigua gravels have lithofacies similar to the glacial outwash braid-plain in the proximal area of the Scott type model. Gravels and sands of the younger Panambi Formation were deposited by a braided stream that was smaller, confined by valley walls, and flowing at a lower gradient than the river that deposited the Antigua gravels. We recognize no sedimentologic characteristics of these deposits as diagnostic of a humid-tropical environment except for textural and compositional changes in matrix sediments caused by deep and rapid chemical weathering.

EFFECTS OF BIOSOLIDS APPLICATION ON GROUND WATER NITRATE-N LEVELS IN SAND AND GRAVEL MINE RECLAMATION IN VIRGINIA

Sand and gravel mine reclamation in eastern Virginia is hampered by low mine soil water holding capacity and fertility levels. Application of biosolids at higher than agronomic rates has been recommended for these areas, but agency concerns over the potential for NO3-N leaching to shallow ground water persist. In the fall of 1998 and the spring of 1999, a mixture of two different biosolids was applied to a 20-ha reclaimed site in the Virginia Coastal Plain at various rates ranging from 1.5 to 5x the agronomic N rate for corn (Zea mays). By the end of the 2000 growing season, the majority of the biosolids treated areas supported at least 90% vegetative cover, and most of the area supported 100% ground cover, with average standing biomass > 5 Mg/ha. Nitrate-N levels remained low in all wells within the treated areas over the winter/spring/summer of 1998/1999, indicating that a mixed application of two biosolids materials to the majority of the site had little effect on ground water with regard to NO3-N. Of 13 monitoring wells under and downgradient of the application areas, only three showed any significant treatment effects, and all levels dropped to < 2 mg/L by the spring of 2001. Nitrate-N levels in several wells directly adjacent to an area receiving a 3x agronomic rate application of lime-stabilized biosolids slowly began to increase in the fall of 1999, peaked at 50 mg/L in late winter/early spring of 2000, and then dropped below drinking water standard levels (10 mg/L) by May, 2000. Neither the surface water within the site nor two external downgradient wells adjacent to the Mattaponi River showed any elevation in NO3-N levels through the late winter of 2001. Significant background levels of NO3-N appeared to be entering the site via ground water flow from an adjacent agricultural field. Overall, these data support earlier findings that while application of biosolids at higher than agronomic rates will lead to an ephemeral (first winter) leaching loss of NO3-N, that the impact to ground water is highly localized, small in magnitude, and relatively short lived. Additional

Delineation of Saltwater Intrusion Through a Coastal Borrow Pit by Resistivity Survey

Dewatering of a borrow pit in Portsmouth, VA, induced localized saltwater intrusion into a sandy coastal aquifer during excavation of the surficial deposit. More than a decade after mining operations ceased, brackish water still contaminates residential wells and the pit lake despite the expectation that recharge would lower the salinity of the wells and the lake over time. Twenty-seven Schlumberger resistivity soundings and two Wenner profiles were taken around the lake and salinity measurements of surface and well water were collected at 27 locations in order to delineate the extent of brackish groundwater in the surficial aquifer. Interpretation of these data, when combined with information from prior limnological and geological studies, indicates the presence of a relatively saline, confined aquifer beneath the lake. This aquifer is connected to the lake only where a deep depression on the lake bottom penetrates through the confining bed overlying the aquifer. The only continuous source of chloride for the lake is brackish water in that depression. Fresh groundwater in the shallow aquifer becomes brackish as it passes through the lake, eventually reentering the aquifer, where it contaminates the residential wells.