Christopher J. Woltemade'

Transcription

1 JULY 1-3 GROUND WATER/SURFACE WATER INTERACTIONS AWRA SUMMER SPECIALTP CONFERENCE 2002 j: 02 L kw TRANSPORT AND FATE OF NUTRIENTS IN A RIPARIAN ZONE: RESTORATION AND MONITORING STRATEGIES Christopher J. Woltemade' ABSTRACT Three distinct hydrologic pathways--stream flow, shallow (alluvial) groundwater flow, and deeper (bedrock) groundwater discharging at a spring-deliver water to an 8.5 hectare riparian zone along Burd Run in south-central Pennsylvania. Eighteen months of water quality monitoring indicate that these separate pathways are an important influence on the fate of nutrients transported through the site. Most significant, deeper groundwater delivers relatively high concentrations of nitrate (mean NO3-N = 7.4 ma) to the stream, which has lower nitrate concentrations (mean NO3-N = 3.8 mgl) upstream of this source. During separate restoration strategies were applied to address each hydrologic pathway and nutrient source: the channelized stream was restored to its historic meandering course, a riparian forest buffer was planted to treat shallow groundwater, and a formerly-drained floodplain wetland was restored to treat deep groundwater discharged at the spring. An ongoing monitoring program assesses the water quality benefits of each strategy. KEY TERMS: water quality, nutrients, riparian, restoration, monitoring, INTRODUCTION Recently, substantial efforts have been made to reduce the delivery of contaminants into surface waters via various watershed "best management practices" (BMPs). In agricultural areas, forested riparian buffers and restored or created wetlands have been widely applied to mitigate the influx of sediments and nutrients into streams and lakes. These approaches have been generally successful, yet the evaluation of numerous projects reveals a wide range of performance, with some projects removing the vast majority of contaminants and others providing only limited benefits (Woltemade, 2000). One of the key influences on the snccess of watershed BMPs is the hydrologic pathway of nutrients and sediments through the site. Individual sites may be differentiated by dominantly surface flow, shallow groundwater flow, deep groundwater flow, or more complex flow paths that combine these routes. Relatively little is known about the efficacy of treatment strategies with respect to the multiple pathways of water and the associated transport and fate of nutrients and sediments through BMP sites (Brenner and Mondok, 1995; Lowrance et al., 1995). Recent research on the water quality benefits of watershed BMPs emphasizes that successful treatment strategies are critically dependent upon site hydrology. Jordan et al. (1993) report that sites with shallow aquicludes beneath riparian forests are highly successful at removing nutrients from groundwater by forcing the flow through the near-surface layers of the riparian forest. Staver and Brinsfield (1990) indicated that little nitrate was removed from groundwater flow beneath the biologically active zone of a riparian buffer. In coastal plain environments, forested riparian buffers have been shown to reduce nitrate-nitrogen concentrations by up to 95% (Lowrance et al., 1984, 1985; Peterjohn and Correll, 1984, 1986; Jacobs and Gilliam, 1985; Jordan et al., 1993). Due to different site hydrology, the removal of nutrients by riparian forests may be less in other physiographic regions (Lowrance et al., 1995) such as the Valley and Ridge region of Pennsylvania, where limited data indicate lower rates of nutrient uptake (Schnabel, 1986). I Associate Professor, Department of Geography-Earth Science, Shippensburg University, 1871 Old Main Drive, Shippensburg, PA Phone: cjwolt@ship.edu. Web 149

2 Agricultural treatment wetlands have effectively removed up to 68% of nitrate-nitrogen and up to 43% of phosphorus (Woltemade, 2000). Reviews of many wetlands indicate a wide range of nutrient removal efficiencies (Kadlec, 1994; Verhoeven and van der Toom, 1990; Watson et al., 1989). This wide range of performance is likely due to differing site-specific characteristics of hydrology an8 wetland design and function, yet these influences remain poorly understood. The influence of retention time on the performance of agricultural wetlands is supported by the comparative performance of wetlands in differing hydrologic conditions (Woltemade, 2OOO; Whigham et al., 1999). As the retention time of water in the wetland decreases, outflow is less completely treated (Jansson et al., 1994). This study examines the transport and fate of nitrate-nitrogen through multiple hydrologic pathways within a riparian zone. The paper has three primary purposes: (1) to present water quality data that illustrate the importance of separate hydrologic pathways contributing water and contaminants to the receiving stream, (2) to describe a comprehensive remediation strategy based on floodplain restoration, and (3) to describe a monitoring program designed to evaluate the success of each watershed BMP at reducing the flux of nitrogen into the stream. STUDY AREA The Burd Run watershed (Figure l), located in south central Pennsylvania, heads atop South Mountain at an elevation of 591 m where two tributaries flow across forested sandy soils developed on Cambrian quartzite. Ne= The Burd Run Watershed Land use 0 Agriculture Forest - 1 2km n- O ~~ Cumberland County, &nsylvania Figure 1. The restoration site, shown at left, is located Pennsylvania. in the Burd Run watershed. south central 150

3 the base of South Mountain, these tributaries flow across thick Pleistocene colluvial deposits that support mixed forestry and agriculture, eventually combining in the Cumberland Valley (Woltemade and Blewett, 2000). Bedrock geology in the valley consists of Ordovician and Cambrian limestones (Root, 1965), some of which contain solution cavities and other karst features (Shirk, 1980). Agricultural land use dominates the lower watershed, typified by thin soils overlying highly permeable carbonate bedrock. Burd Run reaches the floodplain restoration site at an elevation of 190 m, in Shippensburg, Pennsylvania. Agriculture on highly permeable colluvial soils in karst terrain results in high nutrient loading to the interrelated surface water-groundwater system (Lowrance et al., 1995). Due to the karst geology of the lower watershed, surface water-groundwater interactions are complex. The stream is often dry from the base of South Mountain to a limestone spring 700 m upstream of the floodplain study site. Several springs near and within the study site re-establish perennial discharge. The specific focus of this study is an 8.5-hectare floodplain above which Burd Run drains 51.8 km2 (Figure 1). Like many agricultural riparian zones, the site has been greatly impacted by land modification. The native riparian forest was removed in favor of pasture, the stream channel was artificially straightened and a hectare wetland was ditched and partially drained. All of these actions took place prior to 1937, the earliest date of aerial photographs of the site (Hemnann, 1999). Hydrology and water quality At the site, three distinct sources of water can be identified (1) stream channel flows that represent surface flows from the upper watershed (dominant at high flow) combined with groundwater discharge from nearby bedrock springs (dominant at low flow), (2) deep groundwater flow that discharges at a limestone spring within the restoration site, and (3) shallow groundwater flow through a lens of floodplain alluvial cobbles that supplements baseflow. Up to 18 months of weekly water quality monitoring indicate that these waters are chemically distinct (Table 1). Stream flow within upper watershed tributaries draining forested areas of South Mountain is characterized by low nitrate concentrations (mean NO,-N = 1.0 mg/l). At the restoration site, deep groundwater discharged at a limestone spring delivers an highly elevated concentration of nitrate (mean NO3-N = 7.4 mgl) to the floodplain wetland and ultimately Burd Run, which has lower nitrate concentrations above that spring (mean NO3-N = 3.8 mg/l). Testing of nutrient concentrations in shallow groundwater was recently begun under a new monitoring effort (described below). A hydrologically comparable floodplain wetland complex exists m upstream of the restoration site. Here, on-site springs generate surface water from both deep (bedrock) and shallow (alluvial) groundwater sources. Data collected over 13 weeks in winter-spring 2001 also reflect the influence of hydrologic pathway on nutrient concentrations. The deep groundwater source has moderately high nitrate concentrations (mean N03-N = 4.5 mg/l) compared to shallow groundwater that delivers elevated, but lower nitrate concentrations (mean NO,-N = 3.8 mg/l). Table 1. Nitrate-Nitrogen Concentrations in Burd Run Watershed Sites. Site Hydrologic snurce Number of Nitrate as NO.-N Samples Sample Dates Mean Range 1-2 Upper watershed hibutaries 29 6/2000-9/ Burd Run at restoration site 47 6/ I/ Deep groundwater, limestone spring at restoration site 47 6/ I/ Deep groundwater, upstream wetland complex 11 1/2001-4/ Shallow groundwater, upstream wetland complex 11 1/2001-4/ The variation in nitrate concentrations across different hydrologic paths reflects the land use, geology, and hydrology of the source areas of those waters. Stream flows generated from surface runoff in the upper watershed (sites 1-2) have low nitrate concentrations due to the forested land use of that area. In the lower watershed (sites 3-6) the complexity of groundwater flow in karst terrain makes direct associations speculative. 151

4 Nonetheless, it is likely that deep groundwater flowing through bedrock is derived from a relatively large source area with high potential for nutrient loading from agriculture and rural septic systems. Shallow, alluvial groundwater is likely derived from more local source areas. In the vicinity of the Burd Run restoration site and the upstream wetland complex, these local source areas are dominated by residential developments with public sewerage and thus less potential for high nutrient loads. Floodplain restoration A comprehensive floodplain restoration project was completed in (Figure 1). The stream was relocated from the straightened channel to a new 390 m meandering stream channel to reduce excessive bank erosion and improve aquatic habitat and aesthetics. The design of the new channel was based on remnants of the historic (pre-channelization) stream, present-day channel geomorphology of analogous sites, and computer analysis of site hydraulics (Hemnann, 1999). A 25 m wide riparian buffer of native shrubs and trees was planted to remediate the flux of nutrients through shallow groundwater flowing through a layer of coarse alluvium 1-2 meters below the floodplain surface. Constructing a small earthen berm across the drainage ditch increased water retention in the floodplain wetland. This site was identified as critical to the restoration project due to the high nitrate concentrations delivered in deep groundwater discharging into the wetland (site 4) and the high likelihood of substantially lowering nitrate delivery to Burd Run via wetland processes such as denitrification (Woltemade, 2000). PROJECT MONITORING A monitoring project to begin in 2002 will assess the transport and fate of nutrients as they travel through the restored floodplain environment. Water samples will be collected from a total of 45 sites within the restored floodplain (Figure I), providing assessment of deep and shallow groundwater as well as surface water. Weekly water samples will be tested using A " standard methods to establish the concentrations of nitrate-nitrogen, ammonium, and phosphate (Clesceri et al., 1999). Restored wetland The impact of the restored 0.98-hectare wetland on the flux of key nutrients (N and P species) through surface and groundwater pathways will be assessed by sampling: (1) the limestone spring where deep groundwater discharges into the wetland, (2) the wetland's surface outflow, (3) five shallow wells surrounding the wetland, and (4) two transects of piezometer nests located to sample groundwater flowing laterally away from the wetland (Vellidis et al., 1993). One transect will include three piezometer nests and the other will include two nests, each designed to sample groundwater at three depths. The influence of the residence time of water within the wetland on nutrient uptake will he determined by establishing a complete wetland water budget. Riparian forest buffer The impact of the riparian forest buffer on the flux of key nutrients transported through shallow groundwater will be assessed by sampling water from four transects of piezometer nests (see Schultz et al., 1997). These will include the two transects adjacent to the wetland plus two transects of three nests each on the opposite side of the stream (Figure 1). Two additional shallow wells will extend the spatial coverage of the transects, facilitating construction of water table surface maps documenting the relationship of floodplain groundwater with the adjacent stream. Thus, a total of 40 shallow groundwater sampling sites will be established. Stream sites Surface water samples will be collected from the stream channel at three locations: immediately upstream and downstream of the restoration project and within the project site. Additional sites will be sampled to monitor nutrient fluxes into and out of the restoration site, including: (1) deep groundwater discharged at a limestone 1 52

5 spring approximately 700 m upstream of the restored site at the origin of perennial flow in Burd Run, (2) shallow (alluvial) groundwater discharged into the wetland complex upstream of the restored site, and (3) an existing monitoring well approximately 500 m downstream of the restoration site. This well is finished in the limestone aquifer that underlies the alluvial sediments of the restoration site and will provide a control on deep groundwater flowing under the treatment BMF's (Hill, 1996). CONCLUSIONS The delivery of contaminants to receiving waters can he strongly influenced by the various hydrologic pathways through which water flows. In the Burd Run case, streamflows from the forested upper watershed are combined with shallow (alluvial) and deep (bedrock) groundwater sources that deliver elevated concentrations of nitrate. While karst hydrology complicates understanding specific flow paths, it is clear that these nutrient loads reflect the land use and geology of source areas as well as the hydrologic pathway through which the water flows. Accordingly, watershed BMF's aimed at reducing nutrient concentrations should be designed and applied with consideration of site-specific hydrology. Deep groundwater flows through an environment characterized by little biological or chemical activity. As a result, strategies to remediate nutrient concentrations delivered by deep groundwater must focus on treating the water after it re-enters the near-surface biologically active zone, such as with restored or created wetlands. Shallow groundwater often interacts with the biologically active root zone of trees and shrubs; thus riparian forest buffers can contribute to a successful strategy to treat such water. Similarly, monitoring projects assessing the efficacy of such BMPs should be based on a sampling strategy capable of capturing each source or pathway contributing to overall water quality. ACKNOWLEDGMENTS The following individuals assisted with collection and analysis of water samples: Charles Brown IJI, Wendi Hiller, Ian Kramer, and Seleen Shimer. REFERENCES Brenner, F. J. and J. J. Mondok, Nonpoint source pollution potential in an agricultural watershed in northwestern Pennsylvania. Water Resources Bulletin, 31: Clesceri, L. S. A. D. Eaton, and A. E. Greenberg, Standard methods for the examination of water and wastewater, 20'ed. New York American Public Health Association. Herrmann, J., A stream meander restoration study on a small ungauged stream in south central Pennsylvania. M.S. Thesis, Shippensburg University, Shippensburg, PA, 82pp. Hill, A.R., Nitrate removal in stream riparian zones. Jonmal of Environmental Quality, 25(4): Jacobs, T. C. and J. W. Gilliam, Riparian losses of nitrate from agricultural drainage waters. Journal of Environmental Quality, Jansson, M., R. Andersson, H. Berggren, and L. Leonardson, Wetlands and lakes as nitrogen traps. Ambio 23: Jordan, T. E., D. L. Correll, and D. E. Weller, Nutrient interception by a riparian forest receiving inputs from cropland. Journal of Environmental Quality, Kadlec, R.H., Wetlands for wastewater polishing: Free water surface wetlands. In Mitsch, W.J. (ed.), Global wetlands: old world and new. pp Elsevier Science, New York. Lowrance, R., L. S. Altier, J. D. Newbold, R. R. Schnahel, P. M. Groffman, J. M. Denver, D. L Correll, J. W. Gilliam, J. L. Robinson, R. B. Brinsfield, K. W. Staver, W. Lucas, A. H. Todd, Water quality functions of riparian forest buffer systems in the Chesapeake Bay watershed. A report of the Nutrient Subcommittee of the Chesapeake Bay Program. Washington, D.C: U. S. Environmental Protection Agency Report 903-R pp. 153

6 Lowrance, R., R. Leonard, and J. Sheridan, Managing riparian ecosystems to control nonpoint pollution. Journal of Soil and Water Conservation, Lowrance, R., R. Todd, J. Fail, 0. Hendrickson, R. Leonard, and L. Asmussen, Riparian forests as nutrient filters in agricultural watersheds. Bioscience, Peterjohn, W. T. and D. L. Correll, The effect of riparian forest on the volume and chemical composition of baseflow in an agricultural watershed. In Watershed Research Perspectives, edited by D. L. Correll, pp , Washington, D.C.: Smithsonian Institution Press. Peterjohn, W. T. and D. L. Correll, Nutrient dynamics in an agricultural watershed: Observations on the role of a riparian forest. Ecology, 65: Root, S. I., Structural geology of the Cnmberland Valley, Franklin County, Pennsylvania. Pennsylvania Academy of Sciences Proceedings, Schnabel, R. R., Nitrate concentration in a small stream as affected by chemical and hydrologic interactions in the riparian zone. In Watershed Research Perspectives, edited by D. L. Correll, pp , Washington, D.C.: Smithsonian Institution Press. Schultz, R.C., J.P. Colletti, and T.M. Isenhart (eds), Progress report and renewal request: Riparian management system (RiMS) design, function and location. Department of Forestry, Iowa State University, Ames, IA, 153pp. Shirk, W., A guide to the geology of southcentral Pennsylvania Chambersburg, Pennsylvania: Robson and Kaye, Inc. 135pp. Staver, K.W. and R.B. Brinsfield, Groundwater discharge patterns in Maryland Coastal Plain agricultural systems. In Mihursky, J.A. and Chaney, A. (eds.) New perspectives in the Chesapeake system: a research and management partnership. pp Chesapeake Research Consortium, Solomons, MD. Vellidis, G., R. Lowrance, M. C. Smith, and R. K. Hubbard, Methods to assess the water quality impact of a restored riparian wetland. Journal of Soil and Water Conservation, 48(3): Verhoeven, J.T.A. and J. van der Toorn, Marsh eutrophication and wastewater treatment. In Patten, B.C. (ed.), Wetlands and shallow continental water bodies, Volume 1. pp SPB Academic Publishing, The Hague. Watson, J.T., S.C. Reed, R.H. Kadlec, R.L. Knight, and A.E. Whitehouse, Performance expectations and loading rates for constructed wetlands. In Hammer, D.A. (ed.), Constructed wetlands for wastewater treatment. pp Lewis Publishers, Chelsea, MI. Whigham, D.F., T.E. Jordan, A.L. Pepin, M.A. Pittek, K.H. Hofmockel, and N. Gerber, Nutrient reduction and vegetation dynamics in restored freshwater wetlands on the Maryland coastal plain: Final Report. Smithsonian Environmental Research Center, Edgewater, MD. Woltemade, C. J., Ability of restored wetlands to reduce nitrogen and phosphorus concentrations in agricultural drainage water. Journal of Soil and Water Conservation, 55(3): Woltemade, C. J. and W. L. Blewett, 2ooO. Development of an interdisciplinary watershed research laboratory for undergraduate education. In R. W. Higgins (ed.) Water Quantity and Quality Issues in Coastal Urban Areas. American Water Resources Association, Middleburg, Virginia, TPS-00-3, pp