Groundwater contaminants in the deep benthic zone of urban streams in Canada (Invited)
There is little information available on the potential threat that groundwater containing land-based contaminants poses to aquatic ecosystems in urban environments. In this study, a rapid screening approach was applied at the stream reach-scale for eight urban streams (reaches from 100 to < 1000 m). The objective was to determine what types of groundwater contaminants could be detected in the deeper benthic zone of these streams, if any, to start to address questions of whether such contaminants are a concern and which types are the most problematic. The benthic community may be especially at risk since it may experience higher contaminant concentrations than the stream itself due to fewer losses from sorption, degradation and volatilization processes. For each stream, groundwater samples from below the stream bed (typically 25-75 cm) were collected using a drive-point mini-profiler at intervals of 10-15 m along the stream and were subsequently analysed for general chemistry and a wide range of common and emerging urban contaminants. For a few test streams with known contamination, the area of contamination was identified with this technique. In addition, previously unknown contaminants or areas of contamination were identified at all nine streams. Identified contaminants included benzene and other petroleum hydrocarbons, fuel oxygenates (e.g. MTBE), perchlorate, pesticides, artificial sweeteners, and various chlorinated solvent compounds. In addition, elevated levels of nitrate, phosphate, some heavy metals, including cadmium and arsenic, and elevated chloride (likely indicating road salt) were detected. Most streams had many different types of contaminants, often overlapping over small stretches, and together often covering substantial portions of the monitored reach. The findings provide support for this screening approach for delineating areas of potential ecological concern and identifying possible sources of groundwater contamination, for urban settings. They also suggest that the presence of multiple groundwater contaminants may be a more common threat to the benthic community of urban streams than currently perceived.
Urbanization Effects on Low-Order Riparian Groundwater in the Coastal Plain of North Carolina (Invited)
Although much work has focused on stormwater runoff response to urbanization, less is known about the effects of urbanization on riparian groundwater. Rapid population growth and urbanization have increased the potential for hydrological alterations along streams in the southeastern U.S. The study objective was to determine how urbanization affects riparian groundwater hydrology in this urbanizing region. The focus was on low-order streams in small (<5 km2) watersheds in Coastal Plain settings. These streams play key roles in maintaining surface water quantity and quality due to their high drainage density, importance to watershed-scale nutrient cycling, and sensitivity to land-use change. Past studies revealed that urbanization (as quantified by total impervious area (TIA)) in the Coastal Plain of North Carolina has contributed to increased stormwater runoff, which has caused stream channel enlargement and incision along sand-bed channels. In the current study, five low-order streams were selected across a gradient of urbanization (between 4% and 37% TIA) in Greenville, NC. Water levels, stable isotopes (D, O-18), and specific conductivity were monitored in surface water and groundwater during baseflow and storms from August 2007-2008. Groundwater level fluctuations were used to estimate riparian groundwater recharge during rainfall and overbank flow events. Stable isotope composition variations in rainfall, surface water, and groundwater were used to estimate contributions of urban stormwater and groundwater to the streams and to trace rainfall and stormwater inputs to riparian groundwater. Hydrograph separations revealed that urban stormwater comprised an estimated 64% of event discharge at the most urbanized site and 24% at the reference site. Channel incision was greatest (channel depth=2.1 m) at the most urbanized site and resulted in no overbank flow during the year of study in response to increased channel capacity. The reference site (channel depth = 0.8 m) flooded 8 times over the same period and approximately 1/3 of the annual riparian groundwater recharge occurred during overbank events. Riparian groundwater O-18 composition response during storm events also suggested that riparian groundwater recharge decreased with channel incision. Urban channel incision also resulted in increased hydraulic head gradients to the channel. Urbanization affected stream-groundwater interactions by increasing stormwater runoff and channel incision. Consequences were a decline in riparian groundwater levels, reduced riparian groundwater recharge, and a disruption of stream-riparian zone connections. Results suggest that when urban stormwater leads to channel incision, the riparian zone may become less effective at storing floodwaters and enhancing water quality.
Heat and geochemical tracing of contaminated groundwater discharge to streams at various spatial and temporal scales (Invited)
Discharge of groundwater to rivers and streams can be highly variable in space and time and therefore difficult to characterize with the required spatial and temporal resolution. The discharge of highly saline, cool groundwater to Nine Mile Creek, in Syracuse, New York, allowed for comparison between heat and geochemical tracing of surface water-groundwater interaction at multiple spatial scales and under different flow conditions. For geochemical tracing at the reach scale (2 km), we developed two-component mixing models using solute concentrations of stream water and groundwater to determine gross groundwater discharge rates longitudinally along the creek at stream flow rates ranging from 1,320 to 2,250 L s-1. Groundwater in the adjacent aquifer is highly saline, with chloride concentrations of over 50 g L-1 in some locations. At all streamflow rates, the total groundwater discharge over the 2 km reach comprised about 5% of the total streamflow rate at the outlet, indicating that rates of groundwater discharge increase linearly with increases in streamflow. Groundwater discharge occurred predominantly from a localized spring within a 35-m segment of the reach, as indicated by a step change in the stream geochemistry. For geochemical tracing at the bedform scale, in the vicinity of the localized spring, we mapped specific conductance of stream water at the streambed interface to develop qualitative maps of rapid groundwater discharge zones, which corresponded to high measurements of specific conductance (>3 mS cm-1). For heat tracing at the reach scale, we used distributed temperature sensing (DTS) using a fiber-optic cable installed along 900 m of the stream during low-flow conditions (1,400 L s-1). A simple temperature-mixing model indicated groundwater discharge occurred at the localized spring identified through geochemical tracing and groundwater discharge was about 5% of the total streamflow. For heat tracing at the bedform scale, in the vicinity of the localized spring, we used an analytical heat transport model to simulate seepage fluxes across the streambed interface using time-series temperature records. We then developed a rating curve between point-in-time streambed temperature and seepage flux. We determined seepage fluxes at high spatial resolution by applying the rating curve to mapped streambed temperatures (n = 109). Heat tracing indicates a net discharge of water to a 30-m segment of the stream of 1.5 L s-1. The difference between flux estimates from heat and geochemical methods at the reach scale and measurements at the bedform scale is attributed primarily to differences in the spatial extent of the area over which the flux estimates were derived, variability of the groundwater chemistry, and limitations of the heat transport model.
Detection and characterization of local to regional groundwater inputs to rivers, lakes and oceans with electrical imaging (Invited)
Surface water (SW) and groundwater (GW) interact at multiple levels in myriad settings and their interaction is an important hydrogeologic process that impacts ecological and biogeochemical functions. GW discharge and associated mixing with SW in these settings have been challenging to map with sufficient detail and coverage. Three examples are presented on the application of electrical resistivity imaging (ERI) for mapping GW discharge and for understanding SW-GW interactions: (1) a large regulated river, (2) several neighboring lakes, and (3) a fringing coral reef. (1) Time-lapse ERI was conducted at the Colorado River, Texas where the river stage varied by 0.7 m due to dam operations. Submerged and towed electrode cables were used to capture the subsurface mixing dynamics of SW and GW. Using temporal variability in electrical resistivity (ER) signatures, we identified a shallow well-flushed hyporheic zone, a transition zone where SW and GW mix, and a stable deep zone hosting only GW. (2) Towed ER surveys in alkaline lakes in the Nebraska Sand Hills helped distinguishing flow-through lakes, which have decreasing subsurface ER from GW inflow to outflow area, from pure GW discharge lakes, which have uniformly stratified increasing-with-depth ER profiles. (3) More than 30 km of ER profiles collected via towed surveys over a fringing coral reef in the Philippines identified areas of high ER within the reef that coincide with resistive zones in the seawater. Analysis of 222Rn of bottom waters and vertical conductivity-temperature-depth measurements show the persistence of fresh GW input into the ocean where low salinity and high 222Rn areas coincided with high ER areas. A 3D map showing sources and pathways for GW across the reef is developed. ERI is a powerful and convenient tool for mapping GW discharge and SW-GW interactions in rivers, lakes, and oceans.
Exchange processes across sandy beach barriers: Examples from Malibu and Younger Lagoons, California
Estuarine systems in California can manifest themselves as shallow lagoons that are seasonally closed to the ocean by wave-built sand barriers. When a lagoon is physically isolated from the ocean, restricted circulation and sustained material inputs may cause eutrophication, low-oxygen conditions, and persistent algal blooms. During such times, the flow of water and material to the ocean must occur through a beach barrier rather than as surface-water runoff. This subsurface exchange can be modulated by the tides and expressed as a form of submarine groundwater discharge, SGD. Biogeochemically, this transport mode is much different than when a lagoon can exchange freely with the ocean, as redox conditions, organic matter concentrations, water residence times, and salinity can change dramatically. The objectives of this study were to: 1) characterize the seasonal patterns of SGD and associated nutrient loadings in two lagoonal systems that are intermittently isolated from the ocean; 2) assess the physical drivers of this exchange - can we identify the terrestrial versus marine forcing factors and what do these results imply for land / sea exchange along California’s coastline that has many such intermittent coastal systems? Two lagoons in California were studied: Younger Lagoon, an agriculturally-impacted coastal lagoon just north of Santa Cruz, and Malibu Lagoon located north of Los Angeles. Our observations during wet (October 2009 and April 2010) and dry conditions (July 2009) in Malibu captured both open- and closed-barrier scenarios. Lagoon water, groundwater, and seawater were analyzed for 222Rn, salinity, nutrients, DOC, and trace metals during all three field efforts. Initial data and calculations based on radon modeling indicate at least an order of magnitude larger groundwater flux to the lagoon during April 2010 (open barrier) as compared to July 2009, when the barrier was closed. A strong correlation (R2=0.85) between (NO2+NO3) concentrations in surface seawater and salinity during April 2010 revealed that Malibu Creek is the main source of nutrients to the ocean when the barrier is open. In contrast, when the barrier was closed higher total nitrogen concentrations were found in surface water compared to groundwater during low tide. This suggests that sediment in a barrier can provide an ephemeral nutrient source for nearshore seawater. Future work includes barrier pore water profiles at Younger Lagoon to understand biogeochemical nutrient transformations due to groundwater / seawater interactions. Sediment diagenesis and nutrient transformations in such intermittent beach barriers play an important role in evaluating near-shore nutrient budgets.
NITRATE DISCHARGE TO COASTAL WATERS IN RESPONSE TO VARIABLE-DENSITY GROUNDWATER FLOW
Flow dynamics and velocities within coastal aquifers can be complicated by the influence of the saltwater wedge. As a proxy for a number of contaminant types, the variable-density SEAWAT code was used to evaluate the groundwater flow and nutrient fluxes from a coastal aquifer to the sea in response to variable density flow caused by the presence of seawater at the coast. A regional scale coupled variable-density groundwater flow and transport model was developed to characterize and enhance the understanding of complex groundwater flow dynamics, contaminant transport processes, and contaminant flux from the coastal aquifers of southern Baldwin County to coastal surface waters and to the Gulf of Mexico. Simulation results indicate that groundwater flow dynamics and contaminant transport are additionally influenced by density variations that can occur from the incursion of saltwater from the Gulf of Mexico. Residual nitrate concentrations in the saturated zone were estimated to range between 30 and 160 mg/L for the contamination source zones. Simulation results indicate that nitrate concentrations as high as 5 mg/L extend to the deeper Gulf Shores Aquifer. Furthermore, the model indicates that nitrate sources at this depth were released approximately 100 years ago. Vertical and horizontal nitrate transport is attenuated as a result of dilution by dispersion. Simulated nitrate transport trends and concentrations closely resemble the observed ones. Vertical gradients and mixing appear to be significant in this system. The SEAWAT model results reveal the importance of the Intracoastal Waterway in acting as a groundwater and contaminant sink for the Beach Sand and Gulf Shores Aquifers. The model predicts that the Beach Sand and Gulf Shores Aquifers will be impacted by severe saltwater intrusion, whereas the deeper 350 and 500-foot Aquifers will experience no saltwater intrusion for the entire 1,000 year simulation period. Consequently, nitrate discharge to the Gulf of Mexico originates from the lower part of the aquifer system through submarine groundwater discharge. Besides its importance at a regional level, this study represents a basis for decision-making processes in water resources management of other similar coastal aquifer systems of the United States and around the world.
Investigation of Carbon, Nutrients, and Groundwater Inputs in Coastal Florida Using Colored Dissolved Organic Matter
Very few studies of the exchange of water between aquifers and the ocean have been conducted along the Florida coast. Progression of residential and agricultural development in coastal areas is leading to increased nutrients from fertilizers and wastewaters to groundwater. A portion of these nutrients ultimately is released to coastal surface waters. Groundwater mining has increased salt water intrusions in coastal aquifers which may further enhance nutrient fluxes to coastal surface waters. Nutrient concentration in coastal groundwater is sometimes higher than those in river water, counterbalancing for the lower mass flux of groundwater relative to surface waters. Nutrient and carbon inputs through groundwater in certain areas may play an important role in cycling and primary productivity in the coastal ocean. King’s Bay is a spring-fed watershed and manatee sanctuary located on the West Florida Shelf. Over the past 25 years, springs supplying groundwater to King’s Bay have shown a three-fold increase in nitrate concentration and increased invasion of nuisance algae. It has been challenging to track sources of both nutrients and other water quality parameters because there are multiple water supplies to King’s Bay. The goal of this project is to improve the estimate of water, nutrients, and carbon from groundwater discharge into the coastal zone. This paper will present preliminary results of high resolution fluorescence spectroscopy analyses of the various source water types in the King's Bay watershed, including deep and shallow aquifers, wells, springs, and surface water sources. Samples were obtained from various sites--5 springs, 27 wells, 12 surface, and 9 lakes and rivers-- within the King’s Bay area during one dry season. Lakes and rivers had the highest fluorescence intensities and showed similar composition, with the most red-shifted emission maxima. Second highest concentration was seen in some of the wells which had wide range in both composition and intensities. King’s Bay surface sites appear to be a mixture of surface water and spring water based on both composition and concentration. Springs samples were all similar in composition, with concentrations in middle range found in well samples. These results will be discussed in reference to determination of source of water, carbon, and nutrients to the springs.
Application of multivariate statistics and ionic ratio to evaluate seawater and freshwater interaction in small coral island aquifer
Freshwater aquifers in small coral islands of Lakshadweep archipelago occur as thin fragile lenses over the relatively dense saline water. Groundwater is the only freshwater resources for the island population. The study is carried out on Androth, a coral island, one of the 36 coral islands of the Lakshadweep archipelago, in the Arabian Sea off the west coast of India. Saline intrusion is the common problem for sustainability of groundwater resources. Quality of groundwater is significantly influenced by anthropogenic impacts such as unlimited groundwater abstraction due to exponential increase in demand of freshwater for various socio-economical reasons. Hence, groundwater quality monitoring is the need of the hour to sustain the confined groundwater resources. Detail hydrochemical and multivariate statistical interpretations are performed for 12 chemical parameters on samples taken from 35 different dug wells geographically spread over the island. Hydrogeochemical data are subjected to factor analysis such as PCA, Q- mode factor analysis and R- mode factor analysis. Q-mode analysis has been used to decipher the relations among the 35 samples. R-mode analysis has been performed to find the relations among the different variables present in the samples. It is observed that in pre-monsoon season, Cl- alone is mainly responsible for EC and in post-monsoon season EC is combined effect of Na+, Ca2++, Mg2+, SO2-4, Cl-, HCO3-, and NO-3. Besides, the hydrochemical and multivariate analysis, ionic ratios are calculated to delineate seawater intrusion. It includes HCO3/Cl, Na/Ca, Ca/Cl, Mg/Cl and Ca/SO4. The ionic changes are calculated and it is observed that seawater intrusion is a dominant hydrochemical processes. The study shows that there is higher rate of exchange between saline water and freshwater during pre monsoon when compared to post monsoon months.