Michael A. O'Driscoll

Seeps Regulate Stream Nitrate Concentration in a Forested Appalachian Catchment

Surface seeps can be defined as locations where upwelling ground water saturates the surface for most of the year and excess ground water can be delivered to the stream channel via surface flowpaths. If a stream is predominantly fed by seeps, then ground water added to the stream via these surface flowpaths may result in reduced interactions with the subsurface riparian zone. It is generally believed that seep ground water that upwells and then flows along surface flowpaths can be subject to diminished denitrification and biologic uptake processes. Seep effects on stream nitrate (NO(3)) concentration were studied in Baldwin Creek (5.35 km(2)), southwestern Pennsylvania. Nitrate retention within seep zones was evaluated over a 1-yr period (May 2002-2003) using a monthly, nested (top and bottom of seep) sampling approach along 15 individual seeps. Seep samples were analyzed for NO(3)-N, NH(3)-N, and dissolved organic carbon, along with stream waters and streamflow measurements at seven stream stations. Seeps were generally NO(3) sinks with concentrations decreasing downseep: 31% median annual reduction and 73% maximum monthly reduction. During cold and wet periods, seeps frequently behaved as NO(3) sources to the stream (NO(3) concentrations increased or remained constant downseep). Seep temperature and discharge were related to seasonal variability in seep NO(3) retention. Seasonal variations in stream NO(3) concentration have been attributed to upland soil and vegetation processes in numerous watersheds. At Baldwin Creek, seep NO(3) processing regulated the seasonal variability of stream NO(3) concentrations. These results suggest that seeps provide important water quality functions and can modulate the effects of elevated regional N deposition in Appalachian catchments.

Stream Channel Enlargement Response to Urban Land Cover in Small Coastal Plain Watersheds, North Carolina

Stream channel response to urban land use has not been well documented for southeastern Coastal Plain streams. In this study, urban channel response was evaluated in small Inner Coastal Plain watersheds (<5 km2) in eastern North Carolina. Reaches were selected across a range of watershed total impervious area (0-67% TIA). Channel dimensions and sediment grain size data were collected along 20 urban (>10% TIA) and 20 rural reaches (<10% TIA), and at 10 stormwater outfall sites (180 cross-sections). Urban cross-sectional area, channel incision ratio, and channel grain size (gravel%, D50, and D84) were greater, relative to rural channels. Bankfull cross-sectional areas were approximately 1.78 times greater for urban watersheds than for rural watersheds. Channels in urban watersheds were incised and had median full-channel capacities approximately 3.4 times greater than channels draining rural watersheds. Watershed TIA explained 65-72% of channel capacity enlargement. Urban expansion in the region began in the 1960s, with major urbanization occurring over the last 25 years. Channels draining urban watersheds are still responding to this land use change by downcutting and widening. Urban channel incision has frequently cut off streams from their floodplains, reducing floodplain sediment retention and water quality functions.

Controls on groundwater nitrogen contributions from on-site wastewater systems in coastal North Carolina.

The goal of this study was to evaluate the influence of soil type and separation distance to water table on dissolved inorganic nitrogen concentrations in groundwater adjacent to on-site wastewater systems. Groundwater nitrogen species (NO3--N and NH4+-N) and groundwater levels adjacent to 16 on-site systems in three different soil groups (group I- sand, group II- coarse loams and group III -sandy clay loams) were monitored for 15 months (January 2007-March 2008) in coastal North Carolina. On-site systems in soil group I had the highest concentrations of dissolved inorganic nitrogen (median of 18.9 mg/L) in groundwater, and most frequently (mean 61%) exceeded 10 mg/L, followed by systems in soil group II (11.0 mg/L, 50%) and soil group III (2.6 mg/L, 9%), respectively. Groundwater NH4+-N concentrations near on-site systems in soil groups I and II that maintained a 60+cm separation to the seasonal high water table were 4 mg/L lower in relation to systems that had <60 cm separation, but median NO3--N concentrations were 6.5 mg/L higher. On-site systems in group I and II soils are prone to groundwater nitrogen loading with separation distance often controlling the nitrogen speciation in groundwater near on-site systems.

The hydrologic catchment area of a chain of karst wetlands in central Pennsylvania, USA

Shallow depressions found in karst terrains may contain wetlands (karst pans) that fluctuate seasonally in response to climatic conditions. This study examined the ground-water hydrology of a chain of 17 wetlands, ranging in size from 0.06 to 0.4 hectares, located in an Appalachian karst valley in central Pennsylvania, USA. The study objective was to determine the contributing area of wetland source waters. A variety of hydraulic head, soil stratigraphy, and water chemistry data indicate that the combined contributing areas of these wetland ponds extend to a maximum distance of approximately 150 m from the ponds. The hydrologic catchment area of the ponds during January and February, 1999 (approximately 2–8 hectares) was significantly smaller than catchment area based on topography (69 hectares). The water source of the wetlands consists of direct precipitation inputs and shallow (0.5–6 m) perched ground water. The hydrologic catchment area of the ponds expands during wet periods and contracts during dry periods. The ponds have been shown to be perched above the regional water table.

Nitrogen and carbon dynamics beneath on-site wastewater treatment systems in Pitt County, North Carolina

On-site wastewater treatment systems (OWS) are a potentially significant non-point source of nutrients to groundwater and surface waters, and are extensively used in coastal North Carolina. The goal of this study was to determine the treatment efficiency of four OWS in reducing total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) concentrations before discharge to groundwater and/or adjacent surface water. Piezometers were installed for groundwater sample collection and nutrient analysis at four separate residences that use OWS. Septic tank effluent, groundwater, and surface water samples (from an adjacent stream) were collected four times during 2012 for TDN and DOC analysis and pH, temperature, electrical conductivity, and dissolved oxygen measurements. Treatment efficiencies from the tank to the groundwater beneath the drainfields ranged from 33 to 95% for TDN and 45 to 82% for DOC, although dilution accounted for most of the concentration reductions. There was a significant positive correlation between nitrate concentration and separation distance from trench bottom to water table and a significant negative correlation between DOC concentration and separation distance. The TDN and DOC transport (>15 m) from two OWS with groundwater saturated drainfield trenches was significant.

Evaluation of on-site wastewater system Escherichia coli contributions to shallow groundwater in coastal North Carolina

The study goal was to determine if on-site wastewater systems (OSWWS) installed in coastal areas were effective at reducing indicator bacteria densities before discharge to groundwater. Groundwater Escherichia coli (E. coli) densities and groundwater levels adjacent to 16 OSWWS in three different soil groups (sand, sandy loam, and sandy clay loam) were monitored and compared to background groundwater conditions on four occasions between March 2007 and February 2008 in coastal North Carolina. Groundwater beneath OSWWS had significantly (p≤0.05) lower densities of E. coli than septic tank effluent, but significantly higher densities of E. coli than background conditions for each soil type. Twenty three percent of all groundwater samples near OSWWS had E. coli densities that exceeded the EPA freshwater contact standards (single sample 235 cfu/100 mL) for surface waters. Groundwater E. coli densities near OSWWS were highest during shallow water table periods. The results indicate that increasing the required vertical separation distance from drainfield trenches to seasonal high water table could improve shallow groundwater quality.

Geological controls on seasonal-pool hydroperiod in a karst setting

Shallow depressions found in karst terrains may contain temporary (vernal) pools that are inundated seasonally in response to changes in meteorological conditions. The hydrogeology of 16 pools (0.06–0.4 ha) was studied in an Appalachian karst valley in central Pennsylvania, USA. The objective was to determine the effect of the geologic substrate on pool hydroperiod. Meteorological, geophysical, and hydrogeological data collected from November 1997–August 1999 and from January 2002–January 2004 suggested that hydroperiod was primarily controlled by meteorological conditions (total annual precipitation) and surficial aquifer geology. Multiple regression models were found to predict most of the spatial variability of pool hydroperiod with the following variables: thickness of the surficial sandy aquifer; sediment electrical resistivity; and annual precipitation. It might be expected that hydroperiod would be longer for clay pools than sandy pools because clay sediments can act as a seal to perch shallow ground-water and surface-water. Our data revealed the opposite to be true. Sandy residual sediments helped capture infiltration and direct this water along perched ground-water lenses or sheets to seasonal pools. This resulted in annual hydroperiods that were 115 days longer for sandy pools when compared to clay pools. The results suggest that the geologic substrate can be a major control on the duration of hydroperiod.

Groundwater and stream E. coli concentrations in coastal plain watersheds served by onsite wastewater and a municipal sewer treatment system

The goal of this study was to determine if onsite wastewater treatment systems (OWS) were influencing groundwater and surface water Escherichia coli concentrations in a coastal plain watershed. Piezometers for groundwater monitoring were installed at four residences served by OWS and five residences served by a municipal wastewater treatment system (MWS). The residences were located in two different, but nearby (<3 km), watersheds. Effluent from the four septic tanks, groundwater from piezometers, and the streams draining the OWS and MWS watersheds were sampled on five dates between September 2011 and May 2012. Groundwater E. coli concentrations and specific conductivity were elevated within the flow path of the OWS and near the stream, relative to other groundwater sampling locations in the two watersheds. Groundwater discharge in the OWS watershed could be a contributor of E. coli to the stream because E. coli concentrations in groundwater at the stream bank and in the stream were similar. Stream E. coli concentrations were higher for the OWS in relation to MWS watersheds on each sampling date. Water quality could be improved by ensuring OWS are installed and operated to maintain adequate separation distances to water resources.

Nutrient exports from watersheds with varying septic system densities in the North Carolina Piedmont

Septic systems (SSs) have been shown to be a significant source of nitrogen and phosphorus to nutrient-sensitive coastal surface and groundwaters. However, few published studies have quantified the effects of SSs on nutrient inputs to water supply watersheds in the Piedmont region of the USA. This region consists of rolling hills at the surface underlain by clayey soils. There are nearly 1 million SSs in this region, which accounts for approximately 50% of all SSs in North Carolina. The goal of this study was to determine if significant differences in nutrient concentrations and exports exist between Piedmont watersheds with different densities of SSs. Water quality was assessed in watersheds with SSs (n = 11) and a sewer and a forested watershed, which were designated as controls. Stream flow and environmental readings were recorded and water samples were collected from the watersheds from January 2015-December 2016. Additional samples were collected from sand filter watersheds in April 2015-March 2016 to compare to septic and control watersheds. Samples were analyzed for total dissolved nitrogen (TDN) and orthophosphate (PO4-P). Results indicated that watersheds served by a high-density (HD) of SSs (4.9 kg-N yr-1 ha-1; 0.2 kg-P yr-1 ha-1) exported more than double the median masses of TDN and PO4-P, respectively, relative to low-density (1.0 kg-N yr-1 ha-1; <0.1 kg-P yr-1 ha-1) and control watersheds (1.4 kg-N yr-1 ha-1; <0.1 kg-P yr-1 ha-1) during baseflow. Isotopic analysis indicated that wastewater was the most likely source of nitrate-N in HD watersheds. In all other watersheds, isotopic results suggested non-wastewater sources as the dominant nitrate-N provider. These findings indicated that SS density was a significant factor in the delivery of septic-derived nutrients to these nutrient-sensitive, water supply watersheds of the North Carolina Piedmont.