Submarine groundwater discharge and associated nutrient fluxes into San Francisco Bay
We investigated submarine groundwater discharge at two contrasting locations in San Francisco Bay (SFB), the largest coastal system and one of the most urbanized watersheds in the western United States. Two sites, one located in Central Bay and the other in South Bay were sampled for 223, 224Ra, nutrients, and dissolved inorganic carbon in surface waters and groundwater to estimate the amount of submarine groundwater discharge and the associated flux of constituents to bay waters. 223Ra and 224Ra activities were significantly higher in subsurface waters compared to surface waters at both sites. A box model approach of Ra activities along the shorelines of the two sites was used to estimate SGD. SGD, mostly in the form of recirculated seawater, was on the order of 6 to 20 L min-1 per meter of shoreline at the two sampling sites in SFB. Nutrients (NO3-+NO2-, NH4+, PO43-, SiO2) were also higher in groundwater at both sites, except the Central Bay site had higher NH4+ concentrations in surface water. Dissolved organic carbon is higher in bay surface waters than groundwater, which suggests subsurface conditions at some locations may support a natural sink for NH4+ and dissolved organic carbon. Observations suggest that different forms of inorganic nitrogen may be supplied via SGD at the two sites in the bay. NH4+ is being supplied to overlying waters in the South Bay site at 1.4 mmol min-1 per meter of shoreline and NO3-+NO2- is being supplied to the overlying waters at the Central Bay site at 0.23 mmol min-1 per meter of shoreline. The different forms of nitrogen loading likely impact the local near-shore ecosystems, including phytoplankton communities. When extrapolated bay-wide, this data suggests that SGD can possibly contribute significant nutrient loads to the bay surface waters and may impact the overall nutrient ratios compared to other sources to SFB.
Isotopic and Hydrogeochemical Studies on Abnormally High Ammonium of Natural Origin in A Coastal Aquifer-aquitard System
Excessive nitrogen concentration in water bodies is regarded as an environmental contamination because of its possible harm to human bodies and significant ecological effects. Previous studies commonly concerned on elevated nitrogen in water of anthropogenic origins, such as agricultural, domestic, sewage and industrial discharges, because people realize that it is necessary to manage negative influences of human being to the natural environment. Understanding contamination sources of nitrogen is crucial for both waste discharge management and pollutants cleanup. This study was aimed to 1) understand the spatial distribution of abnormally high ammonium groundwater in the Quaternary basalt aquifer in the Pearl River Delta (PRD), China; 2) to distinguish sources of recharge to the basalt aquifer; and 3) to identify the origin of the ammonium in the aquitard and aquifer system. Total 40 boreholes were drilled, and approximately 1000 deposit samples from the aquitard and over 200 groundwater samples from the Quaternary basalt aquifer were collected. A cluster of 7 piezometers was installed in Minzhong Town to study the hydraulic relationships between the aquitard and the basalt aquifer. The results demonstrated that the greater groundwater ammonium concentrations were preserved in the aquifer buried deeper. The ammonium concentration up to 390 mg/L was observed in the basalt sand and gravel Pleistocene aquifer of 20-50m deep, and this is the greatest concentration ever reported for natural groundwater globally. The Quaternary aquitard, which contained abundant sedimentary organic matter and was mainly composed of silt and clay, provided a strict anaerobic environment for sedimentary organic matter mineralization and ammonium preservation. Ammonium concentrations in the aquifer were predominantly controlled by the aquitard ammonium content. This naturally occurring abnormally high ammonium in the Quaternary sediments is areally extensive (over 1600 km2). Great groundwater salinity originated from a marine source redounded to the release of adsorbed ammonium to groundwater via ion exchange processes. This naturally originated high ammonium groundwater may find its way to the river channels and estuary. The flow paths are likely shortened by sand dredging activities Literature shows that no particular studies have been developed for ammonium research in delta aquitards and aquifers. The geological settings of fine texture of delta sediments containing abundant sedimentary organic matter are not unique to the PRD, and this “geological” ammonium may not be an uncommon source of nitrogen and may present as a large and hitherto unappreciated source of nitrogen for surface water bodies.
Submarine groundwater discharge and the coastal ocean extreme bloom incubator Monterey Bay, CA
Intense dinoflagellate blooms occur frequently in Monterey Bay, California. Blooms occur primarily during August through November and can persist for > 1 month. Maximum bloom frequency and mean intensity are in a shallow (< 25 m depth) area of the northeastern bay. This coastal area has been referred to as an ‘‘extreme bloom incubator”. The highest chlorophyll concentrations coincidence with the warmest surface water, low wind stress, and retentive circulation. These coastal physical characteristics favor dinoflagellates, however, it is not clear whether additional chemical constituents contribute to sustaining the seed population that under favorable conditions can develop into a red tide bloom. Here we investigate the contribution of submarine groundwater discharge (SGD) to freshwater, nutrient, dissolved organic carbon and trace metal loads at the “incubator” site in comparison to other sites within the Bay. The potential contribution of SGD to sustaining an ‘‘extreme bloom incubator” is evaluated.
Linking Glaciation and Groundwater on Greenland: Implications for Subsurface Porefluid Chemistry and Sea-Level Rise
Ocean Drilling Program (ODP) Leg 152 discovered relatively fresh and isotopically depleted porefluids between 40- 100km offshore southeastern Greenland. These porefluids suggest sub-ice-sheet meltwater infiltration and associated submarine groundwater discharge. We developed a cross-sectional model for southeastern Greenland using the finite element method to reconstruct sub-ice-sheet recharge and submarine discharge rates, and porefluid salinity and δ18O. We simulated fluid flow and solute transport for ten 100ky glacial cycles. Transient changes in ice sheet geometry were represented using a polynomial function and heat transfer was represented using one-dimensional convection and conduction. We compared porewater chemistry and isotopic composition with ODP porefluids to test the model. We found that permeability and ice sheet thickness had the largest influence on infiltration and discharge rates, and the distribution of freshwater. The extent of the ice sheet and its proximity to the shoreline have the greatest effects on groundwater discharge location. During glacial maximums, submarine discharge dominated over subaerial discharge; however, during interglacials the opposite was true. This study has implications on understanding the contribution of subglacial infiltration, and submarine and subaerial discharge in the transport of Greenland ice sheet meltwater to the ocean. In particular, this study allows for more accurate projections of modern Greenland ice sheet melt rates and contribution to sea-level rise by calculating present subaerial and submarine discharge, which may not be accounted for in surface runoff measurements.
Lake salinity variations resulting from wind direction, Gobi Desert, China
The southern reaches of the Gobi desert, central China, host a large number (~50) of shallow (<3m depth), narrow, north-south trending through-flow lakes. The size of the sand dunes (many over 150m) in this area means that the valleys between the largest dunes can intersect with the water table. The resultant lakes are of particular interest, not only because they are host to a number of unique ecosystems, including several rare species, but also because they are very susceptible to environmental disturbances. Physical development of the lakes is a clear threat, but also small scale withdrawal of groundwater in proximity to the lakes can cause a drop in the water table, forcing it below the lake floor, and consequently causing many lakes to dry up. Due to their inaccessibility, many of these lakes have remained relatively untouched by development, and only those lakes closest to the eastern edge of the desert have been utilized directly for either salt harvesting or tourism. This paper reports on research from both pristine and developed lakes, and reveals a higher TDS (20-50mS/cm compared to 0.5-5mS/cm) in the northern end relative to the southern end for undisturbed lakes. Water entering the southern end of the lakes is chemically identical to the local groundwater (TDS ~0.5mS/cm). This geographic difference in lake properties is remarkable, not only in terms of chemical variation, but also in terms of plant variety and abundance. Stable isotopes show a clear evaporation trend for these lakes, increasing from the southern tip, to the northern tip of individual lakes (-3 to -1‰ in the south, compared with 2-8‰ in the north, and -6 to -3‰ in the groundwater for δ2H). TDS likewise increases with increasing isotopic fractionation. The primary wind direction fluctuates from the southeast to the east, causing the movement of water from the southern end of the lake to the northern, and aiding in the evaporation. Once at the northern end of the lake, the water’s increased density causes it to sink back into the groundwater. In this way, the prevailing wind effectively keeps the fresh and saline waters separate, even though they are part of the same water body. This process is susceptible to even small disturbances. In the developed lakes, this trend is no longer observed, as the system has been interrupted by buildings causing changes to the wind flow, or alternatively, animals and/or human population have altered the water flow, simply through the act of entering the lakes and mixing the lake waters.
Development of a process-oriented conceptual groundwater module for simulation of hydrological processes in meso-scale catchments with shallow aquifers
Groundwater plays a decisive role for the hydrology of catchments which are situated in lowlands, flood plains or pans. Therefore, a number of hydrologic models - numeric, analytic or conceptual - deals with the simulation of groundwater levels and groundwater flow. Often modelling systems use several coupled models to describe the processes in the groundwater, or groundwater levels are only computed in subcatchment scale. This study should introduce a modelling concept for the simulation of groundwater levels within single hydrologic entities and the water fluxes between several adjascent entities. The scope of this conceptual spatially distributed groundwater model is a process-oriented simulation of hydrological processes in meso-scale catchments with shallow aquifers. The integration of the interaction between ground-water and surface water and the description of the processes in and close to lakes or reservoirs are of particular interest. The method presented in the study was applied in the spatially distributed hydrological model J2K and topologically linked hydrologic response units as modelling entities. At first a simple one-to-one routing scheme was implemented. Additionally a new improved multi-flow routing scheme (one-to-many) and the simulation of backwater effects are envisaged for implementation. The underlying equation for the description of the flow processes is largerly similar to the Darcy-equation, whereby some calibration parameters were added. The reliability of the model will be tested with a numerical finite element groundwater model under steady state and transient conditions, observed water table levels in boreholes and lakes and observed river discharge. As test site, the catchment of the Lower Gera River (approx. 540 sqm), which is situated in the Thuringian Basin, was selected for this study. The catchment is defined by the gauges of Ringleben 1 and 2 as catchment outlets, and by the gauge of Erfurt-Möbisburg as catchment inlet. Characteristic elements of the catchment are a wide shallow aquifer consisting of gravel and sand, heavily modified and artificial waterbodies and a cascade of quarry ponds. Due to this complicated and sophisticated layout of the modelling area the subcatchment of the Gramme Creek (approx. 320 sqm), a small tributary of the Gera River, was chosen for calibration of the model. A well observed groundwater monitoring network can be used for the calibration and the validation of the model. The modelling concept is expected to provide an adequate reproduction of the groundwater levels and a realistic estimation of the groundwater fluxes in meso-scale catchments with shallow aquifers.
Bank storage as a thermal sink of temperature surges in urbanized streams
A poorly-studied consequence of bank storage is the ability of the streambed to act as a thermal sink to streams influenced by urban runoff. Headwater streams, with their low thermal inertia, are particularly susceptible. We utilize theoretical calculations and numerical modeling to quantify the amount of heat exchanged with the subsurface during stream temperature surges. We base our study on Boone Creek, a low-order stream in northwestern North Carolina with stream discharge and temperature data dating to March 2006. The catchment is heavily urbanized, and although the stream is of moderate gradient, it is fed by tributaries that lose up to 200 m/km. The combined effect of urbanization and steep gradient produces a flashy response: stream discharge averages 0.10 m3/s, but may increase up to two orders of magnitude during storm events. These events also affect stream and streambed temperatures. Between 2006 and 2008, 33 temperature surges (which we define as >1°C temperature increase in 15 minutes) occurred with a mean temperature increase of 2.6°C and a maximum increase of 6.40°C. Theoretically, heat exchange over a finite reach of the streambed is calculated as f=k(1-n)ΔT/Rd where f is heat flux, k is the thermal conductivity of sediment grains with an average radius of R, n is porosity, d is depth of stream water penetration into the streambed and ΔT is the difference between stream water and pore water. Total heat exchange over a reach of length l in a stream of width w, then, is flw. We model generic storm events based on typical Boone Creek storms and streambed hydrogeology with the U.S.G.S. finite-difference groundwater flow and heat transport code VS2DH. The one-dimensional model domain includes a diurnally-oscillating stream temperature and constant head at the upper boundary, and constant streambed temperature and head at the lower boundary. Reference storm simulations use a temperature increase of 3.66°C and a stream stage increase of 0.66m. Simulations show an increase in streambed temperature of 1.91°C and a one hour lag at a depth of 5cm, which decreases to 0.26°C and a 7.5 hour lag at a depth of 25cm. Storm influence extends to a depth of one meter. Sensitivity simulations suggest that hydraulic conductivity, sediment heat capacity, and thermal conductivity are the most sensitive model parameters. Predictive modeling results suggest further that under reference storm conditions, the streambed matrix absorbs a maximum heat equivalent to 0.64°C, which is then dissipated over the next 6.5 hours. The results of the theoretical calculations and numerical modeling output indicate that streambed sediments may play a role in mitigating high-temperature urban runoff. The results also imply that components of the urban infrastructure that impede groundwater-surface water interaction, such as long culverts, may hamper the natural ability of the alluvial sediments to buffer stream temperatures during temperature surge events.
Long term trend in groundwater levels and watershed condition in the Kurobe River alluvial fan in Japan
The Kurobe River alluvial fan is one of the most popular alluvial fans in Japan. The difference in elevation from Aimoto where is the top of the alluvial fan to seashore is approximately 130m, and the slope of the alluvial fan is approximately 10 degrees. The Kurobe River alluvial fan is consisted of conglomerate layers and has had many flowing wells. Groundwater has been used for domestic and agricultural water in the Kurobe River alluvial fan since a long time ago. In recent years, groundwater usage has been increased caused by the water use for industrial purpose and snow removal operation. National and local governments have installed and observed 19 observation wells in the Kurobe River alluvial fan. Trends in the decrease of groundwater levels were observed in the Kurobe River alluvial fan during 1986-2009. Using groundwater level data at observation wells, these annual and seasonal trends were statistically checked by the Kendall rank correlation test in this study. Moreover, relationships between precipitation, snow depth, land use, river discharge and groundwater levels were investigated using the correlation coefficient. As a result of statistical analysis, groundwater levels at 9 observation wells have been gradually decreasing at significant level 5%. The data and analysis from 2 other wells show that ground water levels have been decreasing significantly at the 1% level. Between the river water level at the Aimoto W.L. station and groundwater levels of observation wells near the river had the strong correlations. Precipitation and snow depth did not show any significant annual/seasonal trend over the Kurobe River alluvial fan. There is not substantial land use/cover change in the Kurobe River alluvial fan. However, the Unazuki Dam gate that is located upstream of Aimoto and used for flood control had been constructed since 1979 and has been operated since 2001. After the dam construction, flood discharge has been decreased drastically in the Kurobe River. Thus, groundwater recharge from the main river might have been decreased due to the dam construction.
Detection of variable groundwater inflow in rivers with geochemical tracers: Using major ion chemistry and radiochemistry to evaluate radon 222Rn as possible tracer, an example from the Avon and Mitchell rivers, southeast Australia
Surface water-groundwater interactions are an important part of the hydrological cycle from ecological and resource perspectives. The dynamics have implications for ecosystems, pollutant transport, and the quality and quantity of water supply for domestic use, agriculture and recreational purposes. Chemical tracers are a valuable tool for understanding the interaction of rivers and the surrounding groundwater. The Gippsland Basin is a significant agricultural area in Southeast Australia. Increasing population has resulted in increased demand of water resources for domestic and agricultural supply. Despite the fact that the Gippsland area receives substantial rainfall, irrigation is still necessary to maintain agricultural production during summer and drier years. The used water resources encompass mostly shallow groundwater and surface water (reservoirs and streams). The effect on the environment range from rising water levels and soil salinisation in the case of irrigation and falling water levels with subsequent necrotization of the vegetation and land subsidence in the case of communal and industrial water extraction. While the surface water components of the hydrological cycle are relatively well understood, groundwater has often been neglected. In particular, constraining the interaction between surface water and groundwater is required for sustainable water management. Gaining and loosing conditions in streams are subject to high temporal and spatial variability and hence, influence the amount of water accessible for agricultural purposes. Following a general assumption recharge to the aquifer occurs during the winter and spring month whereas the river receives water from the aquifer mainly during low flow (base flow) conditions in summer and autumn on a larger scale. Spatial variation, however, are a function of the hydraulic conductivity of the riverbed and the head differences between the aquifer and the river along the river banks. Infiltration and exfiltration rates from changing water levels in the river based on hydraulic models are often underestimated. The hydraulic models do not take into account the complexity of the system and are purely based on discharge figures. Radon (222Rn), stable isotopes and major ion chemistry were used to locate groundwater inputs to the Mitchell and Avon rivers. While stable isotopes and major ion chemistry are useful tracers to determine long-term variability, radon can be used to detect very localised groundwater discharge. Using hydrogeochemistry to locate and quantify groundwater discharge to rivers allows a more accurate assumption on the dynamics of the interaction between surface water and groundwater in the Gippsland area. Radon has been used in similar applications elsewhere. Input parameters for mass balance equations, however, were often approximated and averaged. Radioisotope concentrations in groundwater has been assessed from 20 bores and 5 soil profiles to deliver a more confidential groundwater input water radon concentration by assessing spatial variability and emanation potential of the above-mentioned elements.
In-Situ Pumping Test for Multilayer Hydrogeological Site in Taiwan
Pingtung plain is located the in the southwestern Taiwan, and rainfall concentrated from May to October with the average annual precipitation of 2000 mm. However, topographic steepest rushing stream lead to most of the precipitation becomes runoff and drains to the ocean in short time. Due to the shortage of surface water, the groundwater is an important one of much water recourses. Additionally, the government will be set up artificial lakes in proximal-fan of pingtung plain, which increases recharge and supply to usage of the agricultural and aquaculture. However, the locations of pumping wells are decided not only affecting the developmental quantity of the groundwater but economic growth serious limited. Therefore, MODFLOW-96 was used to simulate the groundwater distribution and optimal the better recharge zone in the regional scale. Based on the model calibration and verification results, the tuku farm of the kaoping lake study is better recharged from Laonong Stream and the northeast, and the safe yield is much than other study zone. Additionally, directional variations in permeability anisotropic formations have important effects on velocities and storage of the groundwater recourses. We have further utilized modifying ANN (artificial neural networks) approach, as well as incorporating the Papadopoulos analytical solution [Lin et al, 2010], to estimate the directional and magnitude of the permeability parameters for pumping test at the tuku farm of the kaoping lake study, which the methodology will be improve accuracy of the estimation parameter. According to drawdown record data of six observation wells, results suggest that the locations of the pumping wells are set up in the northeast and northwest which since sedimentary formations are more permeable along the major direction from the northeast and northwest. Hence, the information can be helpful the groundwater management and supply in the Pingtung plain.