Benoit Lab

Urban watersheds

Urban Bioswales to Reduce Combined Sewer Flows

In this project we are working with multiple partners to construct eight bioswales in a single, small New Haven watershed that is currently served by combined sewers.  We will quantitatively assess both hydrologic and water quality benefits of the bioswales in the treatment and a control site for a period of one year beginning in December 2014.  We will measure total flow as well as several water quality parameters.  We expect the bioswales will achieve three goals:
  1. detain and infiltrate 70% of runoff from the local 1 year recurrence 24 hr rainfall event (6.6 cm in New Haven);
  2. capture 100% of the first flush of 2.5 cm of rainfall in all storms; and
  3. reduce flow amount and the concentration of key contaminants, to significantly decrease loading (mass flux) of contaminants from this watershed to Long Island Sound.
We will use a Before-After-Control-Impact monitoring design (Stewart-Oaten, Murdoch et al. 1986), but with a positive restoration rather than a negative impact as the manipulation. In the treatment sewershed, 8 bioswales will be added at appropriate locations, whereas the control watershed will be left unchanged.  Flow data will be collected for up to 6 months at the outlets of both sewersheds to establish pre-construction conditions, and to confirm that the control behaves similarly to the treatment.  Monitoring will continue for 6 to 9 months after the bioswales are installed, through the end of the funded project.  After bioswales have been constructed and runoff reduction measures implemented, flow through individual bioretention systems will be directly measured, in addition to total discharge at the sub-sewershed outlets. Flow measurement is continuous, so all storms during the study period (one year plus) will be evaluated.
Flow will be monitored by either V notch weirs or tipping buckets.  Monitoring systems will be installed at the inlets of four bioswales in the treatment sub-sewershed.  Total discharge at the outlet of both sub-sewersheds will be monitored by means of Doppler ultrasonic flow meters installed in culverts.  If needed for low flows, where the meters are not adequately submerged, small V notches can be installed temporarily (Thelmar weir or equivalent).  Precipitation will be monitored on site with a tipping bucket rain gauge.  For all sites, swale effectiveness will be evaluated by comparison of storm hydrographs with and without installed GI.  Criteria will include total storm volume, maximum discharge, baseflow level, and water quality.
The water quality parameters we will measure are total suspended solids (TSS), nitrate nitrogen (NO3--N), total nitrogen (TN), orthophosphate (ortho-P), cadmium (Cd) and copper (Cu).  On selected samples, we also will measure fecal coliform bacteria.  It should be noted as well that conductivity (and by extension total dissolved solids, or TDS) and temperature will be measured continuously by all of our level loggers.  TSS is measured because of its multiple undesirable impacts, including smothering of bottom dwellers, clogging of filter feeders, alteration of hard bottom habitat, shading of submerged aquatic vegetation, and transport of particle-reactive toxic contaminants.  Nitrogen and phosphorus forms are of concern because they promote eutrophication in marine and fresh water systems, respectively.  Cadmium and copper are good indicators of persistent toxic contaminants from diverse sources.  Fecal coliform bacteria are indicative of possible pathogens that can sicken humans.  All of these contaminants are known to be removed to some degree by bioswales.
This project is funded by the National Fish and Wildlife Federation and the Long Island Sound Futures Fund.

Evaluation of hydrologic behavior and effectiveness of water quality improvement for constructed wetlands

Excess nitrogen loading, resulting in summertime hypoxia, has been identified as a priority problem facing Long Island Sound.  Sources of nitrogen (N) to LIS include sewage treatment plants and runoff from urban, agricultural, and forested landscapes.  Management plans for reducing nitrogen loading to LIS have called for a 10% reduction in nonpoint (runoff) sources of N in CT and NY.  In the face of continuing development pressures, meeting these targets will require implementation of stormwater best management practices (BMPs) in new and existing urban areas.
Stormwater BMPs are being widely implemented in CT and throughout the country in an effort to alleviate stormwater peaks and reduce pollutant loading to waterways.  Typical structural BMPs include wet ponds, wetlands, dry ponds, infiltration swales, and the like.  However, it is still unclear whether these stormwater BMPs are capable of reducing nitrogen loads to the extent required.  This project aims to assess the efficiency of BMPs in CT and evaluate whether they can provide substantial reductions in nitrogen loads.  This will assist managers in deciding whether (and where) to implement BMPs as nitrogen reduction tools.
Our research focuses on wet ponds and wetlands, which appear to show the greatest likelihood of reducing N loads.  Wet ponds and wetlands can potentially capture nitrogen in several ways:
  • sedimentation of particulate forms of nitrogen;
  • uptake of soluble nitrogen by aquatic or wetland plants (ultimately leading to nitrogen burial in sediment);
  • denitrification of NO3 (leading to loss as N2 gas).
Our overall objective is to understand the effectiveness of constructed wetlands and wet ponds in removing nitrogen from stormwater in the LIS watershed.  Furthermore, we hope to identify key factors that contribute to BMP effectiveness in the Connecticut region.  We are quantifying how influent N level, effluent pathway (infiltration or surface overflow), and water residence time affect the amount of removal.  We are also comparing N removal effectiveness in storms of various sizes and evaluating differences that are likely to occur on a seasonal basis.  We are evaluating if there is a floor below which N levels cannot be lowered (the “irreducible concentration”) and we are determining its value.  By obtaining all of this information, we should be able to provide guidance for watershed managers on how to size, design, and locate BMPs for maximum effectiveness in reducing N loading to Long Island Sound.
In the course of this research, we are testing the following hypotheses:
  1. Stormwater ponds in the LIS watershed are able to achieve statistically significant reductions in stormwater N loads.
  2. N removal is greatest when influent concentrations are high, and may be limited or non-existent when they are low.
  3. N removal efficiencies vary seasonally, being higher in the growing season when loading to Long Island Sound is most critical.
  4. N removal increases with water residence time within the BMP and with extent of infiltration.
  5. Assessing N removal efficiency requires knowledge of BMP hydrology.
We are studying eight BMPs, measuring N removal effectiveness in nearly every storm over a period of approximately one year.  For each storm event, we are measuring flow proportional concentrations of nitrate (NO3-), total nitrogen (TN), and chloride (Cl-) in stormwater influent and effluent.  We are also measuring flow amounts by continuously monitoring water level (stage) and establishing stage-discharge rating curves, usually with V-notch weirs that we install.  For most storms, we composite all samples, weighted to flow, to derive a single average value, the so called Event Mean Concentration (EMC).  On a subset of storms, we separately measure individual samples collected every 30 minutes over the period of storm flow.  These time series measurements allow us to better understand the detailed dynamics of N delivery and removal.  The amount of infiltration in each BMP is being calculated by means of a hydrologic mass balance, where infiltration is derived as the difference between addition in measured inflow and precipitation, less losses in measured outflow plus evaporation.  Chloride, as a conservative tracer, is used as a check on hydrology by confirming that there is zero removal in the BMPs.
Evaluating the N removal efficiency of BMPs is a complicated task.  Simply measuring the concentration of N species in influent and effluent can be misleading for several reasons.  First, N levels tend to change over the course of a storm, so many samples need to be measured to characterize each storm properly.  Second, flow rates also change, and concentrations at higher flows will have a greater contribution than those at lower flows.  Thus, concentrations need to be weighted in proportion to flow to give a representative result.  Third, water exiting the BMP at a moment in time generally entered the BMP much earlier, maybe even during a previous storm event.  Thus, changing N levels over the course of a storm could be misinterpreted as removal or production within the BMP.  Fourth, some small storms may not fill a BMP to capacity and produce no effluent at all.  Finally, in unlined BMPs, a significant amount of water can be lost to infiltration.
This project was funded by NOAA - Connecticut Sea Grant

The Yale Experimental Watershed (YEW): Design and evaluation of interdisciplinary restoration of an abandoned urban open space site

F&ES faculty and students have long desired to develop teaching, research, and ecosystem management opportunities on Yale University’s campus.  This has been realized in what has come to be called the Yale Experimental Watershed (YEW), a 19,7 acre (8 ha) forested area whose central section is in the city block across the street from FES’s main buildings (Sage and Kroon Halls).  For hundreds of years, a small stream has flowed through the site.  In the last century, the site comprised a mix of partly wooded backyards maintained by Mansfield and Prospect Street residents, but it has been largely unmanaged for several decades.  Over the years, Yale acquired most of the properties within the project area, and the desire to better control storm water and provide teaching and research opportunities on campus has created an interest in managing the site in aggregate.
The goal is to transform the YEW from an underutilized and degraded site to one that is highly productive – where academic research can be conducted and community members can recreate.  Funding was granted by the Hixon Center for Urban Ecology for a site assessment.  The Center has guided this process over the past three years by selecting and supporting students under the direction of several faculty collaborators.
Work so far has been extensive and varied.  Forty ground sampling plots were established based on a geometric grid with each plot placed 80 feet (25 m) from the next.  All the trees ≥ 4 inches (10 cm) diameter were mapped with GPS, classified to species level, and their height and diameter at breast height were recorded. Herbaceous and ground cover vegetation were characterized within 40 circular sampling plots, each having an area of 4 m2.
Soil samples were collected from all 40 vegetation plots and analyzed for litter depth, bulk density, pH, carbon and nitrogen, substrate induced respiration (SIR), water holding capacity, gravimetric moisture content, and macronutrients (Ca, Mg and K and P).  Coarse woody material was recorded and classified via line intersect sampling to estimate the number, volume, and class of coarse woody debris in the area.  This information is useful for characterizing habitat in forested systems.
In hydrological studies, a two-foot contour map of the site was used to delineate and map the YEW. At present, surface runoff from precipitation on 4.3 ha of the watershed area (54% of the area) drains to the storm-water drainage system and never reaches the undeveloped portion of the YEW. 
Twelve monitoring wells in two transects were drilled to monitor ground water levels and to try to estimate subsurface flows.  They are being monitored continuously (water depth, conductivity, and temperature).  To monitor surface hydrology, 30° V-notch weirs were installed at the inlet and outlet of the site, as well as on a pipe draining a portion of the Prospect-Sachem parking garage and its associated upland area.  Continuous monitoring of water depth in the weirs has been carried out since March 2013, providing reliable discharge data.  The site is also instrumented with a meteorological station, solar radiation meter, and soil moisture probes.  These data, in conjunction with parallel measurements of total dissolved solids, will permit calculation of water and salt balances.  Evapotranspiration will be estimated based on the standard Penman-Monteith model, which uses the meteorological data, and independently via soil moisture measurements. 
Preliminary results show that flow into the site occurs year round, whereas surface outflow stops temporarily at the height of the growing season.  In the spring, surface inflow was less than outflow, indicating that groundwater is an important contributor to the water budget
Already, real-time data on water quality and hydrology in YEW are being made available on two websites ( and and other kinds of monitoring data will be added soon.
In terms of management, once we have a good understanding of the site’s hydrology, we plan to conduct a study to see how reconnecting drainage from impervious surfaces can beneficially alter the water balance of YEW.  Another simple intervention would be to cut the arboreal vines that are stressing many of the larger trees on the site.  We also hope that YEW will be used as a site to test the effect of various land management strategies.
This project is supported by the Hixon Center for Urban Ecology.

Development of an End Member Mixing Analysis multi-tracer method to identify nonpoint pollution sources.

This research develops a tracer method that will enable ecosystem managers to identify and apportion nonpoint sources of pollution.  This knowledge can be invaluable in directing scarce resources towards improving water quality.  The proposed research is efficient, since it adapts a well-established method currently used for identifying natural chemical sources and flow paths in pristine environments, expanding its application to pollution sources across the landscape.  The goal is to demonstrate the utility of end member mixing analysis (EMMA) to identify and quantify the relative contribution of multiple nonpoint sources to pollution loads of streams draining Connecticut watersheds.  Sources (end members) include precipitation, dry deposition, septic tanks, leaking sewage lines, street drainage, lawn chemicals, roof runoff, agriculture, and weathering.  Most of the proposed tracers are measured routinely, to make the method as widely applicable as possible.  Additional novel tracers are being identified and evaluated to increase the method’s reliability.
Twenty-nine tracers have been measured and evaluated in the eight end member sources and several impacted streams.  The tracers are SO42-, Cl-, Na+, K+, Ca2+, Mg2+, SiO2, HCO3-/ANC, DOC, conductivity, Br-, I-, F-, Li+, Rb+, Sr2+, H2VO4-, MoO42-, H3BO3, Ni, Cu, Zn, UO2(CO3)22-, ClO4-, Pd, caffeine, ibuprofen, 35S, and PAHs, though only a best subset (≈ 12) of these will be used in the final tracer method.  (Many of the tracers can be measured simultaneously, so the number of analyses is fewer than the list might suggest.)  In addition, two contaminants (NO3-, Hg) are measured to evaluate the tracer method’s ability to identify their sources.  Data synthesis includes principal component analysis (to determine the rank of the data), multiple linear regressions, explicit consideration of uncertainty in end member composition during regression, backward estimation of end members, alternative testing, and sensitivity analysis to identify the best subset of tracers.
Supported by NOAA - Connecticut Sea Grant Program

Sediment export from watersheds in St Thomas, USVI: Erosion, transport, and redeposition.

Coral reefs and other vulnerable nearshore marine resources of the Virgin Islands are suffering from the impacts of multiple stressors.  Among the most important of these is fine-grained sediment eroded from land and delivered by streams to the coastal zone.  Steep slopes, erodible soils, widespread soil disturbance (for land development and road building), and inadequate use of erosion prevention and control measures lead to accelerated erosion and sediment export.  Short distances and steep slopes mean that Virgin Islands watersheds are closely coupled with adjacent marine ecosystems.  Corals must clear themselves of deposited sediments to obtain prey and, more importantly, to allow symbiotic zooxanthellae to photosynthesize.  Even non-deposited sediments can shade corals, reducing photosynthetic efficiency.  Dealing with fine sediments requires expenditure of unnecessary energy, which when combined with predation, elevated temperatures, disease, predation, and physical disturbance can cause coral morbidity and mortality.  A better understanding of erosion, transport, and delivery of fine-grained sediments would inform better management to protect fragile coral reef ecosystems from the impacts of onshore land development.
This study was designed to answer several research questions:
  • How does fine sediment export during small rain events compare to that in larger storms?  Can small pulses associated with minor storms pose a disproportionate risk to reefs because of their frequency?
  • How does the amount of fine sediment export under baseflow conditions compare to that in storms of various intensities?
  • How does fine sediment export change over the course of individual storms?  For example, is there a “first flush” effect?
  • What is the relationship between rainfall amount, rainfall duration, time since antecedent rain event, and season (wet/dry) and fine sediment export?  Does the exponential increase in sediment flux with discharge that has been observed in some other systems apply on St Thomas?
  • Do coastal pond sediments reveal changes in sediment accumulation rates during the last century, and if so, can they be linked to historical events or changes in land use?
Our goal was to continuously monitor discharge and suspended matter in Dorothea Gut (the local name for streams) for a period of at least a year.  This was facilitated by installing a V notch weir as an addition to an existing broad crested weir built by the USGS.  The V notch allows sensitive measurement of small discharges as stage declines.  We deployed a YSI 6920 Multi Parameter Water Quality sonde in Dorothea Gut by attaching it in the pool behind the weir.  The sonde was equipped with probes to measure pressure (stage/water depth), turbidity (a surrogate for suspended sediment), temperature, dissolved solids, dissolved oxygen (DO), and pH.  Starting in June 2009, we collected data every 15 minutes, except for brief periods when the sonde was returned to the lab for service and recalibration. 
Sediment cores were collected from ponds along the coast of St Thomas.  Geologically, these systems tend to be small embayments whose connection to the sea has been closed by some combination of reef building and longshore drift creating berms rising 1 to 2 m above sea level.  Because they lack direct connection to the sea, they act as settling basins and capture a share of the sediment delivered to them by guts during storms.  We identified six ponds with suitable watersheds for this study: North and South Perseverance Ponds, Red Hook Pond, Fortuna Pond, Compass Point Pond, and a small upland pond above Sapphire Beach (we refer to it as Sapphire Pond).  We have also sampled an embayment connected to the ocean, Mandahl Bay, as well as Mangrove Lagoon.  We collected 2 to 4 cores from each pond.  Sections from the cores were analyzed for 210Pb, 137Cs, 226Ra, and 7Be.  Taken together, these radionuclide measurements provide detailed information on sediment accumulation over the past 100 years, as well as the extent of mixing processes occurring near the sediment water interface.
An important and consistent trend in the data is the response of the gut to rain events.  When it rains, water depth (and discharge) increases as does turbidity (and SPM), while TDS decreases because of dilution.  TDS in the gut is very high, typically 1.4 g/l, similar to a solution that is 4% seawater.  Rainwater here is 0.1 g/l or less.  The most likely explanation for the elevated TDS is high evapotranspiration, which removes water but leaves salts behind.  This dramatic difference between the rain and the stream means that TDS dilution might be used to estimate the increase in discharge for comparison to results based on stream stage.
Evaluating the six ponds measured so far, sediment accumulation rates (SARs) are low.  Looking at the ponds, we calculate delivery rates, assuming 100% capture, near 100 kg ha-1 yr-1, except for Sapphire Pond, which is about 10 times higher.  These low values compare favorably with those measured on St John for hillslope plots (10 – 270 kg ha-1 yr-1) and zero and first order catchments (10 and 80 kg ha-1 yr-1) by Ramos-Scharron and MacDonald (2007). 
Considering the steepness of the slopes and level of development, we expected SARs to be higher.  The low SAR values we measured imply either that (1) the ponds are very inefficient sediment traps, (2) erosion is minimal, or (3) transport of eroded material is negligible.  We believe that mechanisms 2 or 3 are the most likely explanations for the low SARs.  (1) Based on pond and watershed characteristics, the ponds should be able to contain most of the inflow they receive, except in very large storms.  Water residence times thus being high, and settling depths short, the ponds should be efficient sediment traps.  (2) Much of the watershed area of these ponds is undisturbed, so most land can be expected to produce little eroded material, despite their steep slopes.  (3) Much of the steep uplands above these ponds are upslope from undisturbed, vegetated lands.  They should not generate significant overland flow that could carry eroded material.  Thus, even if significant erosion does occur, much of the material is likely to be redeposited before it can reach the ponds.
The management implication of these results is important.  Erosion from undisturbed watersheds on St Thomas appears to be naturally low.  The nature of soils, vegetation, and climate is such that little material is eroded and that which is mobilized is quickly redeposited despite steep slopes.  This means that small areas of disturbed soils, mainly associated with land development, can contribute a vastly disproportionate share of sediment to downslope ecosystems.  On the positive side, because these developed areas tend to be small (typically the size of building lots), erosion control measures can be employed cost effectively.  Also, because there is an obvious link between the erosion source and an individual developer or builder, responsibility is clear and enforcement should be straightforward.
Funded by the Water Resources Research Institute

Interdisciplinary study of before and after conditions in a coastal wetland undergoing restoration

This project has been monitoring improvements in environmental quality that are occurring as the result of restoration of a well-studied salt marsh ecosystem.  The West River estuary between West Haven and New Haven, Connecticut, was tide gated for nearly a century, creating a degraded fresh-tidal system with reduced water quality and biodiversity.  A local NGO obtained major funding to install self-regulating tide gates that remain open except during rare flood events.  This restoration re-established salt water flushing that dramatically increased tidal range, water quality, and salinity.  These improvements in the physical characteristics of habitats will almost certainly lead to beneficial changes in plant and animal communities.  We are quantifying environmental conditions with high temporal resolution monitoring of the system’s hydrology and water quality.  These key drivers will have a substantial effect on vegetation, fish, and bird communities, which we are monitoring in parallel research.  Importantly, the marsh should respond quickly enough to make an experiment possible in only a few years.   The restoration is a unique opportunity to assess ecological responses in a large scale Before-After-Control-Impact (BACI) experiment.  Data were collected before and are continuing to be accumulated after restoration of tidal exchange and the results compared to quantify the impact of the restoration.  As a control, the research protocol is being replicated at a similar system, located within the same city, but which is not being restored.  Both the experimental and control marshes are located in urban park areas.  The restoration can be expected to provide benefits to neighbors and park users as well as to wildlife.  There are many ecosystem restoration projects taking place worldwide, but very few of them are adequately monitored to see if they are successful and how they affect wildlife populations and human communities.
We deployed YSI 6920 Multi Parameter Water Quality sondes in the treatment and control sites for more than a year before the tide gates were modified.  The sondes were equipped with probes to measure pressure (stage/water depth), salinity, turbidity (a surrogate for suspended sediment), temperature, dissolved oxygen (DO), and pH.  Starting in 2010, we collected data every 15 minutes, except for brief periods when the sonde was returned to the lab for service and recalibration.  Results show strong similarity of these two time series indicating that the Mill River is a suitable control for the West River.  The prevailing tidal range (caused mainly by fresh water backing up behind the closed tide gates) is nearly identical, as is absolute depth, and both exhibit an asymmetrical tide, with the flood limb taking significantly longer than the ebb.  Another important feature is that both estuaries respond similarly to high river flows, for example, rain events.  When the tide gates were opened, the physical and chemical parameters responded instantaneously in the treatment site, the West River.  The figure shows a brief period in November when the tide gates were closed for repairs.  Note the dramatic difference between the control and treatment before and after the temporary closure, and the similarity and abrupt transition when the gates were closed and reopened.
This project was sponsored in part by the YSI Foundation.
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