Water is both an abundant and scarce

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1 15 Universities Council on Water Resources Issue 139, Pages 15-21, April 2008 Land-Use Impact on Water Pollution: Elevated Pollutant Input and Reduced Pollutant Retention Weixing Zhu 1, Joseph Graney 2, and Karen Salvage 2 1 Department of Biological Sciences, 2, Department of Geological Sciences and Environmental Studies, Binghamton University Water is both an abundant and scarce resource; less than one percent of water on Earth is available to terrestrial and fresh water ecosystems, including all human usage. As the renewable source of fresh water, 110,000 km 3 is precipitated on land annually, an average of just 70 cm/yr across the globe. Rapid human development, including urban and agricultural activities, demand a large and reliable supply of clean fresh water. Currently, over half of accessible fresh water runoff has already been appropriated for human use, and the issue of water supply will become more urgent in this century due to growing demands from human societies and uncertainty associated with global climate change. Non-point source pollution is a particular challenge to environmental management due to diverse sources of pollution and multiple and often complicated pathways of pollutant transport in the landscape. For example, non-point source pollution of nitrogen (N) and phosphorus (P) causes widespread eutrophication in the nation s rivers, lakes, estuaries, and coastal oceans, leads to algal blooms, oxygen depletion in water, fish kills and loss of native biodiversity (Carpenter et al. 1998, Howarth et al. 2000). The N and P pollution comes from diverse sources, including fertilizer usage in agricultural fields and suburban lawns, manure from animal feedlots, urban runoff, and atmospheric deposition (Carpenter et al. 1998). That makes quantitative predictions of how pollutants move from sources to sinks and possible management choices very difficult. The watershed approach is a powerful way to address non-point source pollution because a watershed can be considered a naturally-bounded hydrologic system, where water and pollutant inputs can be clearly delineated and their outputs from the watershed can be monitored. A watershed is not only the site of a hydrologic basin study, but also the location at which all ecosystem processes are occurring. In small headwater watersheds, experimental approaches to manipulate watershed structures and functions, such as cutting trees to examine the changes of stream runoff and water chemistry, have greatly increased our understandings of biological and hydrological controls on watershed processes (Likens and Bormann 1995, Swank and Crossley 1988). At large drainage basin scales, budgetary approaches to quantify total watershed inputs, many associated with anthropogenic activities linked with total watershed outputs, have provided important information on how human activities increase nonpoint source pollution and downstream exports of the pollutants from the watershed (Howarth et al. 1996, Boyer et al. 2002). Pollutant Inputs into the Watershed One of the major causes of water pollution is land use. For example, N and P inputs in agricultural fields and urban lawns greatly increase the N and P pollution in agricultural and urban watersheds (Carpenter et al. 1998). In the Chesapeake Bay Watershed, N outputs in streams from 17 headwater watersheds in the Coastal Plain Province and 10 watersheds in the Piedmont Province were found to be positively correlated with the proportion of cropland in the watershed (Jordan et al. 1997a, b). Average annual N discharge was estimated at 18 kg/ha from cropland and just 2.9 kg/ha from forestland in the coastal plain area (Jordan 1997a). entire.indd 15 3/17/ :38:55 AM

2 16 Zhu, Graney, and Salvage In the Piedmont area, average annual N discharges were 42 and 1.2 kg/ha from cropland and forestland, respectively. In Baltimore County, Maryland, also located in the Chesapeake Bay Watershed, N annual pollutant export through streamflow was more than 10 times higher in a suburban watershed than in a forested watershed (6.5 vs kg N/ ha), while that from an agriculture watershed was 30 times higher (Groffman et al. 2004). In urban ecosystems, in addition to fertilizer usage, a large amount of N can be imported in foods for humans and their pets and N fixed in automobiles and power plants can be deposited near the emission sources. Zhu et al. (2006) for example, found that nitrate concentrations of surface soils in the Phoenix urban ecosystem varied by four orders of magnitude, with soils from different types of urban land use containing significantly higher nitrate-n (sometimes more than ten times higher on average) than the native desert soils. In southeastern New York, Pouyat and McDonnell (1991) provided convincing evidence that significantly more heavy metals were accumulated in forest soils in the urban end of an urban-rural gradient. Pollutants generated outside the watershed boundary can be deposited through atmospheric deposition. The northeastern U.S., for example, has the highest rates of N deposition on the North American continent, ranging from 5-20 kg N per hectare annually, which is 5-10 times the preindustrial background. The pattern of N deposition here shows clear trends of west-east decline and south-north decline, pointing strongly to industrial sources in the western and urban sources in the southern portions of the region (Ollinger et al. 1993). Boyer et al. (2002) estimated that atmospheric deposition was the largest single source of anthropogenic N input in northeastern U.S. watersheds (31 percent), followed by the inputs in food and feed (24 percent), N fixation in agricultural fields (24 percent), fertilizer use (15 percent), and N fixation in forests (5 percent). Pollutant Removal in Watersheds Land-use change not only affects pollutant input in a watershed, but also affects its hydrology. In a forested watershed, precipitation is absorbed by the tree canopy, lost to the air through evapotranspiration, percolates through the soil layers, and recharges the ground water that feeds stream base flow. Surface runoff in agricultural watersheds is usually much higher than in forested watersheds and even higher in urban watersheds, where the percentage of impervious surface is extremely high. Urban hydrology can be further altered due to reduced evapotranspiration (because of lower vegetation cover) and engineered storm water systems that directly channel surface runoff to streams. The altered hydrology leads to enhanced peak runoff during storms, leading to flooding. Impervious surface can also dramatically alter the pattern of pollutant output from the watershed because of hydrologic effects on pollutant delivery and on the biogeochemical retention and removal of pollutants. The hydrologic flow path in a watershed (from precipitation to stream flow) affects the contact time between pollutants, biological systems (vegetation, surface and vadose zone soil, riparian area along streams, etc), and biogeochemical processes that could retain or remove pollutants. When hydrological flow is fast and contact time is low, such as in spring snowmelt and summer storms, pollutants can be carried to streams with minimal removal. The difference in flow path can be either anthropogenically or naturally caused. In the previous example (Jordan et al. 1997a, b) the Piedmont Province of the Chesapeake Bay Watershed has much steeper slopes than the coastal plain province and thus much faster hydrological flows. In forested watersheds, annual N outputs from both areas were small (averaging 1.2 and 2.9 kg/ha respectively). In agricultural watersheds, however, the much larger annual N pollutant export from the Piedmont area (averaged at 42 kg/ha) dwarfed the export from the Coastal Plain (averaged at 18 kg N per hectare), even with similar land use and similar N input, strongly suggesting the effects of natural hydrological variations on pollutant export. The contact between pollutants carried by hydrologic flow and biogeochemical processes that retain or remove them (e.g., denitrification) usually intensifies in riparian zones of the watershed. That is why the riparian area needs to be protected for the benefits of pollutant removal and preventing direct pollutant discharges into streams. The distribution of riparian wetlands, however, can be constrained entire.indd 16 3/17/ :38:55 AM

3 Land-Use Impact on Water Pollution 17 by the geographic setting of the watershed. In the Chesapeake Bay Watershed, the Coastal Plain has the highest distribution of riparian wetland, while the Piedmont and the Valley and Ridge areas have much lower riparian coverage due to steep slopes (Lowrance et al. 1997). In urban watersheds, highly engineered storm water drainage systems often short-cut the filter function of the natural riparian wetland by discharging water directly into the stream. Furthermore, the altered hydrologic flow pattern, characterized by elevated peak discharge, has been causing enhanced stream channel erosion and other alterations of stream geomorphology. A resulting consequence is the lowering of water tables in nearby wetlands, further diminishing their function of removing non-point source pollutants (Groffman et al. 2003). Studies in a Small Watershed at Binghamton University Over the past several years, we have studied a headwater watershed where the State University of New York-Binghamton campus is located (Figure 1). The 8 km 2 watershed is drained by a perennial stream (Fuller Hollow Creek) that discharges directly into the Susquehanna River, the largest tributary of the Chesapeake Bay Estuary. The watershed contains forested areas, including a 180-acre Nature Preserve, suburban land use, and urban land use characterized by dense human populations, a large percentage of impervious surface, and high daily traffic flow on the campus. We sampled stream water chemistry at multiple locations along the creek, including tributaries draining sub-watersheds dominated by natural, suburban, and urban land uses. We also sampled a storm water retention wetland in one of the urban sub-watersheds. The study was conducted in the Fall in conjunction with Watershed Hydrology and Watershed Ecology classes partially supported by the National Science Foundation education programs and in the Summer through a special intern program supported by the National Institutes of Health. Thus, we integrate environmental scientific Figure 1. Aerial photo of 8 km 2 Fuller Hollow Creek Watershed. Watershed delineation shows the suburban (Sub), natural (Nat), and urban (Urb and Urb2) sub-watersheds. Sampling locations are indicated as circles and include one at main channel and others at the tributaries. Sampling locations within the Urb subwatershed were before (Urb) and after (UrbR) a retention wetland. Susquehanna River is in the northeast corner of the photo. entire.indd 17 3/17/ :38:55 AM

4 18 Zhu, Graney, and Salvage Figure 2. Stream nitrate concentrations in the autumn of (a) 2004, (b) 2005, and (c) 2006, showing the effect of land use on water pollution and the remediation effect of the urban retention wetland. Sub: suburban land-use; Nat: forested drainage subbasin; Urb: urban drainage subbasin before a retention wetland; UrbR: urban drainage basin after a retention wetland; Main: main stream channel; Urb2: 2nd urban subbasin. research with environmental education, and apply our findings to environmental management and public outreach (see Graney et al., this issue). Studying land use effects in a single headwater watershed also allows us to minimize natural geological, biological, and climatic variations often occurring on larger-scale projects. Variations in stream chemistry showed a clear pattern of the effect of land use on water pollution (Figure 2). Nitrate concentrations in tributaries draining urban sub-watersheds were 5-10 times higher than those draining suburban and natural sub-watersheds. In Fall 2004, we found nitrate concentrations in an urban tributary passing through a storm water retention wetland (UrbR) were much lower than another urban tributary without any retention facility (Urb2, Fig. 2a). That led us to ask the question: could retention wetlands, often used in urban storm water management, also be used to reduce water pollution? Since riparian wetlands are rare in the urban part of the watershed due to deep incision of the creek channel, this watershed management option for reducing pollutant loads is very important. In 2005 and 2006, we sampled the urban tributary both upstream and downstream of the retention wetland. Our data clearly showed that in the months of September and October, and in both 2005 and 2006, stream nitrate concentrations were significantly reduced after passing through the wetland (Fig. 2b, c). The mitigation effects of retention wetlands on urban runoff can be linked to its chief function of mitigating storm water flow (Fig. 3). The attenuated flow condition in the retention wetland increases the contact time between flow that carries upland urban pollutants, and wetland plants and sediments, allowing both biological uptake and biogeochemical processes to occur. However, the detailed hydrologic and biogeochemical interactions remain to be determined, and our ongoing research is addressing these issues. Management Choices and Conclusions From our biogeochemical and hydrologic studies in a headwater watershed in the upper Chesapeake Bay Watershed and the literature review, it is clear that both elevated pollutant inputs associated with changing land use and altered hydrologic flow entire.indd 18 3/17/ :38:56 AM

5 Land-Use Impact on Water Pollution 19 patterns have large impacts on water pollution. Management decisions using sound watershed science principles are needed to reduce pollutant load, prevent direct pollutant discharge into surface and ground water, and increase pollutant removal. Urban hydrologic alterations in particular, by either reducing the area of riparian wetland or diminishing its biogeochemical function, make this natural filter of non-point source pollution less functional in urban watersheds. In regions where the natural distribution of riparian wetlands is limited, storm water retention wetlands can both attenuate the peak discharge as well as reduce the pollutant export to streams. Other management decisions, such as reducing impervious surface cover in the watershed, increasing storm water infiltration to the vadose zone and allowing it to follow natural hydrologic flow paths from uplands to wetlands to stream channels, would also be useful, not only in controlling peak storm discharge, but also in Figure 3. Stream hydrographs showing the attenuation of peak stormflows before (Urb) and after (UrbR) an urban retention wetland (top) and the difference between a natural sub-drainage basin (Nat) and an urban sub-basin (Urb2, bottom) in August entire.indd 19 3/17/ :38:57 AM

6 20 Zhu, Graney, and Salvage reducing pollution discharges. Further interdisciplinary studies are needed to fully understand the hydrological, biogeochemical, and ecological interactions occurring in watershed ecosystems, including retention wetlands. Where should the retention wetlands be located and how big should they be? Do pollution reductions vary according to hydrologic conditions as well as seasonally? From human perspectives, flooding controls are likely more important to local residents, yet management choices can effectively incorporate pollution controls for the benefit of both local residents and those living hundreds of miles away downstream. The health of natural terrestrial and aquatic ecosystems in the watershed also stand to benefit. Acknowledgements We would like to thank Binghamton University Provost s Inter/Multidisciplinary Symposia Program which supported this Watershed Symposium and this special issue and funding support from the NSF CCLI grant and EPA Watershed Initiative for our campus watershed study. We greatly enjoyed the colleague support from Binghamton University s Center for Integrated Watershed Studies and enthusiastic participation of many undergraduate and graduate students in the research. Author Bios and Contact Information Weixing Zhu, Ph.D., is an associate professor in biological sciences at the State University of New York at Binghamton. His research focuses in the areas of nitrogen biogeochemistry and ecosystem ecology. His address: Dept. of Biological Sciences, Binghamton University, Binghamton, NY wxzhu@ binghamton.edu. Joseph Graney, Ph.D., is an associate professor in the Department of Geological Sciences at the State University of New York at Binghamton. His research focuses on environmental geochemistry and surface water hydrology. His address: Dept. of Geological Sciences, P.O. Box 6000, Binghamton university, Binghamton, NY jgraney@binghamton. edu. Karen Salvage, Ph.D., is an associate professor in the Department of Geological Sciences at the State University of New York at Binghamton. Her research focuses on ground water hydrology and modeling. Her address: Dept. of Geological Sciences, P.O. Box 6000, Binghamton university, Binghamton, NY ksalvage@binghamton.edu. References Boyer, E. W., C. Goodale, N. A. Jaworski, and R. W. Howarth Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A. Biogeochemistry 57/58: Carpenter, S., N. F. Varoco, D. L. Correll, R. W. Howarth, A. N. Sharpley, and V. H. Smith, Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8: Groffman, P. M., D. J. Bain, L. E. Band, K. T. Belt, G. S. Brush, J. M. Grove, R. V. Pouyat, I. C. Yesilonis, and W. C. Zipperer Down by the riverside: urban riparian ecology. Frontiers in Ecology and the Environment 1: Groffman, P. M., N. L. Law, K. T. Belt, L. E. Band, and G. T. Fisher Nitrogen fluxes and retention in urban watershed ecosystems. Ecosystems 7: Howarth, R. W., G. Billen, D. Swaney, A. Townsend, N. Jaworski, K. Lajtha, J. A. Downing, R. Elmgren, N. Caroco, T. Jordan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch, and Z.-L. Zhu Regional nitrogen budget and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35: Howarth, R., D. Anderson, J. Cloern, C. Elfring, C. Hopkinson, B. Lapointe, T. Malone, N. Marcus, K. McGlathery, A. Sharpley, and D. Walker, Nutrient pollution of coastal rivers, bays, and seas. Issue in Ecology No. 7, Ecological Society of America, Washington, DC. Jordan, T. E., D. L. Correll, and D. E. Weller. 1997a. Effects of agriculture on discharges of nutrients from coastal plain watersheds of Chesapeake Bay. Journal of Environmental Quality 26: Jordan, T. E., D. L. Correll, and D. E. Weller. 1997b. Nonpoint source discharges of nutrients from piedmont watersheds of Chesapeake Bay. Journal of the American Water Resources Association 33: Likens, G. E., and F. H. Bormann Biogeochemistry of a Forested Ecosystem. 2 nd edition, Spinger-Verlag: New York. Lowrance, R., L. S. Altier, J. D. Newbold, R. R. Schnabel, P. M. Groffman, J. M. Denver, D. L. Cornell, J. W. Gilliam, J. L. Robinson, R. B. Brinsfield, K. W. entire.indd 20 3/17/ :38:57 AM

7 Land-Use Impact on Water Pollution 21 Staver, W. Lucas, and A. H. Todd Water quality functions of riparian forest buffers in Chesapeake Bay watersheds. Environmental Management 21: Ollinger, S. V., J. D. Aber, G. Lovett, S. E. Millham, R. G. Lathrop, and J. M. Ellis A spatial model of atmospheric deposition for the Northeastern U.S. Ecological Applications 3: Pouyat, R. V., and M. J. McDonnell Heavy metal accumulations in forest soils along an urban-rural gradient in Southeastern New York, USA. Water, Air, and Soil Pollution 57-58: Swank, W. T., and D. A. Crossley, Jr. (eds.), Forest Hydrology and Ecology at Coweeta. Springer- Verlag: New York. Zhu, W.-X., N. Hope, C. Gries, and N. B. Grimm Soil characteristics and the accumulation of inorganic nitrogen in an arid urban ecosystem. Ecosystems 9: entire.indd 21 3/17/ :38:57 AM