2012 S.C. Water Resources Conference - Exploring Opportunities for Collaborative Water Research, Policy and Managemen

Callahan, Timothy J

Epps, Thom H

Griffin, Michael P

Hitchcock, Dan R

Vulava, Vijay M

English

Clemson University: TigerPrints

Using Water Chemistry Data to Assess Stormwater Pathways in Lowland Watersheds  Michael P. Griffin1, Thom H. Epps2, Timothy J. Callahan3,  Vijay M. Vulava3, Dan R. Hitchcock2  AUTHORS: 1Environmental Studies Graduate Program, College of Charleston, 66 George Street, Charleston, SC 29424; 2Baruch Institute of Coastal Ecology and Forest Science, Clemson University, Georgetown, SC 29440;  3Dept of Geology and Environmental Geosciences, College of Charleston, 66 George Street, Charleston, SC 29424  REFERENCE:  Proceedings of the 2012 South Carolina Water Resources Conference, held October 10-11, 2012 at the Columbia Metropolitan Convention Center 	  	   ABSTRACT.  Forested lowland watersheds of the lower coastal plain (LCP) are being developed for residential, commercial, and industrial use at an increasing rate. We explored the use of end-member mixing analysis (EMMA), a hydrochemical technique, to better understand water flow processes in the LCP.  We compared results from chemical hydrograph separation (CHS) from EMMA and a physical hydrograph separation (PHS) approach based on falling hydrograph limb regression. These approaches were applied at Watershed 80 (WS-80), in the Santee Experimental Forest near Cordesville, SC, which drains 200 hectares.  Samples from water-table wells, piezometers, lysimeters, rain gauges, and streamwater samplers were assessed for major cation and anion concentrations, which serve as natural chemical tracers.   Both methods provide estimates of storm-event flow: quickflow by PHS and rainwater contribution by CHS. Results indicate a high level of consistency between the approaches for some storms.  However, other storms show markedly differing results.  These results illustrate the complexity of runoff production dynamics in LCP watersheds and suggest that using parallel methods provides a more nuanced understanding of hydrological processes. Our findings suggest that a rough understanding of groundwater flow processes can be accomplished for low-order lowland watersheds in periods of less than one year using a rapid deployment of EMMA.  Due to the shallow water table, piezometers can be installed by hand-auguring. Lysimeters and rain gauges are inexpensive and easily installed.  Streamwater samples can be collected manually, or preferably with a sampler.  Sample processing can be contracted to an academic or private laboratory, and data processing can be performed with free software.  These findings show that a hydrogeochemical approach to understanding lowlands watersheds is not cost- or time-prohibitive, and can provide critical information to land managers and policymakers who oversee the urbanization of these watersheds.   INTRODUCTION   The Lower Coastal Plain (LCP) of the United States is undergoing dramatic and rapid change.  Development is converting forested, minimally developed, and rural landscapes into urban, suburban, and exurban areas.  Key changes that result from such development include: removal of vegetation, covering of bare earth with surfaces of limited permeability, and modification of stream channels; all potentially result in increased stormwater runoff.   From 1973 to 1994, urbanized land use in the Berkeley/Charleston/Dorchester tri-county region grew by 256%, even though population during this time only grew by 41% (BCDCOG, 1997).  Allen and Lu (2003) predicted that between 2000 and 2030, urbanized land use in the tri-county region will triple.  Unless significant mitigation efforts are made, between 2000 and 2025, an additional 866 miles of streams statewide are likely to be in areas where they will suffer severe degradation, that is, where impervious land cover is greater than 10% (Exum et al., 2005).  As land is developed, forested lands are lost.  This is a particular concern in the southeastern USA, since forecasts indicate that 90 percent of forested land use loss in the United States from 1997 to 2060 will occur in the East, and more than half in the South (Wear, 2011). To better understand the effect of change on these watersheds, it is critical to understand how these watersheds function in their natural state.  However, the hydrologic processes of forested coastal lowland watersheds in the southeastern U.S. are not fully understood (Kirchner, 2003; Winter et al., 2003; Amatya et al., 2006).  Geologic, topographic, climatic, and meteorological factors combine to make estimations of runoff in response to storm events difficult.   Physical hydrological approaches for studying hydrological processes and water-resource issues have long been employed.  In this study, we explore the use of a hydrochemical technique to give a fuller understanding of groundwater/surface-water interactions and stormwater pathways in a minimally developed, first-order watershed.  We hypothesize that using physical and chemical approaches in tandem will give the clearest understanding of these subtle processes.  This information is critical for developers and water-resource managers in an era of rapid urbanization.  PROJECT OBJECTIVES        In this paper we will demonstrate the use of natural water chemistry to elucidate hydrological processes of coastal-plain watersheds through the use of chemical hydrograph separation; compare results from chemical hydrograph separation to a physical hydrograph separation technique, and thereby describe benefits of both approaches; and describe an approach for a rapid deployment to study water chemistry at a small watershed.  METHODS Site Description The Santee Experimental Forest (SEF) is a minimally developed and lightly managed forest within the Francis Marion National Forest near Charleston, SC, and has been used for science and management research since 1937 (Amatya et al., 2003).  The control watershed of the SEF, WS-80 is a first-order watershed with a small, ephemeral, channelized stream.  The soils are primarily clays and loams.  Historically used for rice cultivation, WS-80 is now composed of a forest canopy of pine-hardwood (39%), hardwood-pine (28%) and mixed hardwoods (33%) (Amatya and Radecki-Pawlik, 2007). The site is instrumented for thorough hydrological and hydrochemical analysis.  Shallow water-table wells were used to track the depth of the water table throughout the year.  Piezometers of varying depths allow for pressure (aquifer head) assessment and sample collection at specific depths below the ground surface.  Soilwater suction lysimeters (Soilmoisture Equipment Corp., Santa Barbara, CA) collected water from the vadose zone by vacuum pressure.  Streamwater auto-fraction collectors from Isco (Teledyne Isco, Inc., Lincoln, NE) collected periodic stream samples.  Automatic data logger modules (Solinst Canada Ltd, Georgetown, ON) recorded water table position hourly. A weather station (Campbell Scientific, Inc., Logan, UT with CR10X data loggers) was operated by the US Forest Service Center for Forested Wetland Research in Cordesville, SC, and included an automatic tipping bucket rain gauge to track rainfall quantity and storm intensity.  Manual rain gauges (Forestry Suppliers Inc., Jackson, MS) measured and captured rainfall and forest-canopy throughfall.  	  Figure 1.  Watershed 80 (WS-80) at the Santee Experimental Forest, 30 miles northeast of Charleston, SC.  This watershed is instrumented with water-table wells, piezometers, suction lysimeters, rain gauges, a auto-fraction stream-water sampler, and a weather station.  This infrastructure is identified by site numbers, shown in the image above as white numbers, from high to low elevation: 1-3, B, 27, 23, and 4. Site 1 is at N33.1491, W79.7889.  Stream flow was calculated from stage height at a compound outlet weir less than 1 km downstream from the stream sampling location, at the watershed outlet.  The stream ponds upstream of the weir, causing a muted hydrograph effect, as flow slows when it joins this ponded area.  Flow rate is calculated using an established stage-discharge at this weir by the US Forest Service and was a good estimate of stream flow from the watershed (Harder et al., 2007).   Sampling and data collection occurred on a roughly monthly basis from 2009 through 2011 and quarterly during 2011.  Sampling before and after storm events was a priority.  Physical Hydrograph Separation (PHS) Using a process developed for the coastal plain (Williams, 2007), quickflow and baseflow quantities were estimated based on regression analysis.  This approach assumed storm flow and sustained baseflow to be two linear reservoirs discharging at different rates. Baseflow was modeled according to Maillet’s (1905) single linear reservoir discharge model: Q(t) = Q0 * e(-t/k).  Linear regression of log(Q) estimated baseflow for a given storm event.  Flow in excess of baseflow was assigned as storm flow, or quickflow.  In the absence of quickflow, the logarithm of stream flow decreased linearly.  If the logarithm of flow was decreasing non-linearly, there was quickflow contributing to the stream.  This method gave estimates of quickflow with only stream flow data.  For full treatment of this method, see Rogers (2010) and Epps et al. (2012).  Chemical Hydrograph Separation (CHS) Water samples were filtered with 0.45 µm Millipore-HP MPES syringe filters, and prepped by standard methods for analysis on an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500cx, Agilent Technologies, Santa Clara, CA) for determination of total concentrations of Ca, Na, K, Al, Mg, Si, Fe, and Mn. ICP-MS detection limits were less than 0.10 ppm.  Samples were also run on an ion chromatograph (IC, Metrohm, Riverview, FL) for anion concentration determination, including F−, Cl−, Br, NO3-, PO43-, and SO42- using standard methods (Eaton and Franson, 2005).  IC detection limits were less than 0.50 ppm. To elucidate data trends, principal components analysis (PCA) was performed using open-source software R (The R Foundation for Statistical Computing, http://www.r-project.org).  PCA was performed on the stream samples, to study how natural tracers change over time, and medians of potential water sources (or end-members) were transformed based on the PCA results.  The PCA results allow for choosing end-members that contribute to stream flow, such as upland groundwater, riparian groundwater, streambed groundwater, soilwater, and rainwater. Using the PCA results, end-member mixing analysis (EMMA) was performed (Christophersen and Hooper, 1992; Burns et al., 2001; Garrett et al., 2012).  A series of linear equations were solved for each stream sample, using Mathematica 8 (Wolfram Research).  Solving these equations for each stream water sample, the fraction of total flow from each end-member was calculated throughout storm events.  With the resultant data, hydrographs were constructed with the end-member contributions separated, to show the relative influence of each end-member during the study period.  RESULTS AND DISCUSSION  Comparing Physical and Chemical Hydrographs 	  Quickflow, as determined by the PHS method, was roughly analogous to the rainwater end-member of CHS.  Baseflow by PHS was roughly analogous to the other CHS end-members combined.  Results indicated a high level of consistency between the methods for some storms, such as the February 2011 storm shown in Figure 2.  However, other storms, such as the September 2010 storm shown in Figure 3, showed differing results for quickflow by PHS and rainwater by CHS.  The variability may have resulted from differing assumptions made about baseflow and storm flow in the LCP.  By the graphic method, quickflow was determined by discharge timescale, whereas by the chemical method, the storm flow contribution was determined by how closely the stream water chemistry mimics rainwater.  In the case of the September 2010 storm event, the large variation in the methods was likely caused by the very large rainfall volume on very dry conditions. The initial rainfall infiltrated and subsequent rainfall during the storm event was interpreted as baseflow by the physical method (because it was interflowing to the stream as baseflow typically does) but as quickflow/rainwater by the chemical method (because its residence time was short, Figure 2.  Physical Hydrograph Separation (left) and Chemical Hydrograph Separation (right) for the 2/4/2011 storm event.  Baseflow from PHS is compared with the sum of shallow riparian water and upland groundwater from CHS. 	  and thus its chemistry was like rainwater).  These results illustrated the complexity of runoff production dynamics in LCP watersheds and suggest that using parallel methods provides a more nuanced understanding of hydrological processes.  Implementing Water Chemistry Analysis Our findings indicated that analysis of natural water chemistry can provide information about hydrological processes that physical approaches alone cannot supply.  For a fuller understanding of water resource issues at a site, we recommend supplementing traditional physical hydrological studies with limited water chemistry analysis. EMMA-based chemical hydrograph separation is a cumbersome process, but our results suggest that a streamlined approach is possible in order to do a relatively rapid assessment of a watershed.  The following recommendations form the procedures for such an approach.  In lowland watersheds, the water table is shallow, even in dry conditions, so the following piezometers can be installed by hand-augering: • An upland piezometer, collecting groundwater from 3-5 m below ground surface (bgs). • A riparian piezometer, collecting groundwater from 2-3 m bgs. • A streambed piezometer, collecting groundwater from 1-2 m bgs, or above the argillic horizon, if present. • A streambed piezometer, collecting groundwater from 3-5 m bgs. • A streambed stilling well. Stage height measurements are needed, which can be recorded by a data logger (Solinst Canada Ltd., Georgetown, ON) in the streambed stilling well.  Our results show the importance of assessing unsaturated-zone soilwater, which is accomplished using inexpensive suction lysimeters.  Rainwater can be easily collected in manual rain gauges.  For rough estimates, daily streamwater samples may be adequate for profiling storm-event stream chemistry.   Water samples could be sent to a private or academic laboratory for analysis.  In situ analysis of some values, such as chloride and conductivity, may be feasible with modern field equipment. Statistical analyses can be performed in free, open-source software R (www.r-project.org), EMMA can be performed in the freely available website WolframAlpha (www.wolframalpha.com), and data can be compiled and hydrographs can be created in free, open-source software OpenOffice.org (www.openoffice.org). It is our belief that a rough assessment of a site, using the above recommendations, can offer significant information about storm-event runoff at a site pending development in less than a year.  The results can help to inform the specific stormwater practices implemented during development to manage stormwater quantity and quality issues.  ACKNOWLEDGMENTS  This research was conducted in collaboration with D.M. Amatya of the US Forest Service at the Santee Experimental Forest, Cordesville, SC.  The authors acknowledge Mr. Andy Harrison, hydrological technician at the US Forest Service, for provision of meteorological and stream-flow data.  The authors thank T.M. Williams of Clemson University for his collaborations and helpful discussion on this project. Finally, the authors thank other professionals and students who assisted with this project: C. Guinn Garrett, Nicholas Wallover, Brooke James, Lydia Nickolas, Brad Sion, Ryan Rembert, and Sam Kuzma. Figure 3.  Physical Hydrograph Separation (left) and Chemical Hydrograph Separation (right) for the 9/27/2010 storm event.  	  This work was prepared as part of a work sponsored by the S.C. Sea Grant Consortium with NOAA financial assistance award NA06OAR4170015. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of SC Sea Grant or NOAA.  LITERATURE CITED     Allen J., Lu K. 2003. Modeling and prediction of future urban growth in the Charleston region of South Carolina: a GIS-based integrated approach. Conservation Ecology 8: 2.  Amatya D.M., Miwa M., Harrison C.A., Trettin C.C., Sun G. 2006. Hydrology and water quality of two first order forested watersheds in coastal South Carolina.  Amatya D.M., Radecki-Pawlik A. 2007. Flow Dynamics of Three Forested Watersheds in Coastal South Carolina. Press: Acta Scientiarium Polonorum–Formatio Cirtumiectus.  Amatya D.M., Sun G., Trettin C.C., Skaggs R.W. 2003. Long-term forest hydrologic monitoring in Coastal Carolinas.  27-30. BCDCOG. 1997. Twenty year time series analysis of satellite remote sensor data for environmental and developmental change along Coastal Georgia and South Carolina. Final report submitted to NASA via CD ROM.  Burns D.A., McDonnell J.J., Hooper R.P., Peters N.E., Freer J.E., Kendall C., Beven K. 2001. Quantifying contributions to storm runoff through end-member mixing analysis and hydrologic measurements at the Panola Mountain Research Watershed(Georgia, USA). Hydrological Processes 15: 1903-1924.  Christophersen N., Hooper R.P. 1992. Multivariate analysis of stream water chemical data: the use of principal components analysis for the end-member mixing problem. Water Resour. Res 28: 99–107.  Eaton A.D., Franson M.A.H. 2005. Standard methods for the examination of water & wastewater. Amer Public Health Assn. Epps T., Hitchcock D., Jayakaran A., Loflin D., Williams T., Amatya D.M. 2012. Characterization of storm flow dynamics of headwater streams in the South Carolina lower coastal plain. J. Amer. Wat. Res. Assoc: In press, submitted 06 April 2012.  Exum L.R., Bird S.L., Harrison J., Perkins C.A. 2005. Estimating and projecting impervious cover in the southeastern United States. EPA Rep. No. EPA/600/R-05 61.  Garrett C.G., Vulava V.M., Callahan T.J., Jones M.L. 2012. Groundwater–surface water interactions in a lowland watershed: source contribution to stream flow. Hydrological Processes. 10.1002/hyp.8257. Harder S.V., Amatya D.M., Callahan T.J., Trettin C.C., Hakkila J. 2007. Hydrology and water budget for a forested Atlantic coastal plain watershed, South Carolina. Journal of the American Water Resources Association 43: 563-575.  Kirchner J.W. 2003. A double paradox in catchment hydrology and geochemistry. Hydrological Processes 17: 871-874.  Rogers D. 2010. A Study of Coastal Headwater Stream Hydrology: Bannockburn Plantation, Georgetown County, SC. M.S. Thesis, Clemson University. Wear D.N. 2011. Forecasts of county-level land uses under three future scenarios: a technical document supporting the Forest Service 2010 RPA Assessment. Notes.  Winter T., Rosenberry D., LaBaugh J. 2003. Where does the ground water in small watersheds come from? Ground Water 41: 989-1000.    

Using Water Chemistry Data to Assess Stormwater Pathways in Lowland Watersheds

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Using Water Chemistry Data to Assess Stormwater Pathways in Lowland Watersheds

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