Identifying groundwater recharge connections in the Moscow (USA) sub-basin using isotopic tracers and a soil moisture routing model

Globally, aquifers are suffering from large abstractions resulting in groundwater level declines. These declines can be caused by excessive abstraction for drinking water, irrigation purposes or industrial use. Basaltic aquifers also face these conflicts. A large flood basalt area (1.1?×?10⁵ km²) can be found in the Northwest of the USA. This Columbia River Basalt Group (CRBG) consists of a thick series of basalt flows of Miocene age. The two major hydrogeological units (Wanapum and Grand Ronde formations) are widely used for water abstraction. The mean decline over recent decades has been 0.6 m year^?1. At present day, abstraction wells are drying up, and base flow of rivers is reduced. At the eastern part of CRBG, the Moscow sub-basin on the Idaho/Washington State border can be found. Although a thick poorly permeable clay layer exists on top of the basalt aquifer, groundwater level dynamics suggest that groundwater recharge occurs at certain locations. A set of wells and springs has been monitored bi-weekly for 9 months for δ¹⁸O and δ²H. Large isotopic fluctuations and d-excess values close to the meteoric water line in some wells are indicating that recharge occurs at the granite/basalt interface through lateral flow paths in and below the clay. A soil moisture routing (SMR) model showed that most recharge occurs on the granitic mountains. The basaltic aquifer receives recharge from these sedimentary zones around the granite/basalt interface. The identification of these types of areas is of major importance for future managed-aquifer recharge solutions to solve problems of groundwater depletion.     

The Effect of Submerged Aquatic Vegetation Expansion on a Declining Turbidity Trend in the Sacramento-San Joaquin River Delta

Submerged aquatic vegetation (SAV) has well-documented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAV on the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from long-term monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21–70 % of the total declining turbidity trend is due to SAV expansion.     

The levelized cost of carbon: a practical,if imperfect,method to compare CO2 abatement projects*

Calculating the cost effectiveness of projects and policies with respect to reducing carbon emissions provides a simple way for local government agencies to consider the climate impacts of their actions. Yet, defining a metric for cost-effectiveness in relation to climate change is not straightforward for several reasons. In this paper, we focus primarily on dynamics, reflecting the time value of money and how the benefits of reducing carbon emissions may change over time. We define a cost-effectiveness metric called Levelized Cost of Carbon (LCC) that carefully accounts for these dynamics. We also investigate the theoretical and practical implications and limitations of using a cost-effectiveness metric as an approach to rank projects. We apply our metric to a set of transportation projects to illustrate the insights that can be gained by such a process.

Key policy insights:

• Levelized Cost of Carbon (LCC) provides a simple way for local governments to consider climate change mitigation in decision making.

• LCC is a cost-effectiveness metric that carefully accounts for the time value of money and possible changes in the value of reducing emissions through time, thus helping local governments to make better decisions.

• LCC can be used to rank projects, with some caveats, even in the absence of a specific value for the benefits of reducing GHG emissions, thus providing flexibility in the face of uncertainty and political constraints.

     

Variation in groundwater salinity in a tidal salt marsh basin,North Inlet Estuary,South Carolina

Groundwater flow of freshwater from upland forests into salt marshes is influenced by hydrologic forces that operate over a wide range of temporal scales, including storm events, tidal fluctuations, seasonal variations in rainfall and evapotranspiration (ET). Groundwater salinity can be a useful first order indicator of the balance between these flow processes. A dipole-dipole electrical resistivity survey was conducted approximately monthly during 2005 to measure groundwater salinity across a portion of Crabhaul Creek, a tidal salt marsh basin at the boundary of the upland forest and the North Inlet marsh in South Carolina. The monthly electrical resistivity surveys were designed to provide a detailed, spatially continuous measurement of subsurface conductivity to a depth of 4 m in order to further investigate the seasonal variation in groundwater salinity. Resistivity models were corroborated by simultaneous measurements of salinity in nearby piezometers. The freshwater-brackish water interface was clearly imaged by the resistivity. Movement of this interface occurs on a timescale of months rather than a regular seasonal variation. The average salinity in the marsh basin is highest in late Summer (July–August) when ET is highest, and lowest during the Winter (November–December). The position of the brackish-freshwater interface changes, but is not well correlated to local rainfall or tidal cycles except under specific circumstances. A steady-state hydrological model correctly predicts the average position of the freshwater-brackish interface and suggests a linear relationship between the height of the water table and the location of the interface. These results suggest a complex relationship between precipitation events and groundwater flow from the forest into the marsh.     

Bioaugmentation in a Well‐Characterized Fractured Rock DNAPL Source Area

A field demonstration was performed at Edwards Air Force Base to assess bioaugmentation for treatment of a well‐characterized tetrachloroethene (PCE) dense nonaqueous phase liquid (DNAPL) source area in fractured rock. Groundwater recirculation was employed to deliver remedial amendments, including bacteria, to facilitate reductive dechlorination and enhance DNAPL dissolution. An active treatment period of 9 months was followed by a 10‐month posttreatment rebound evaluation. Dechlorination daughter products were observed in both the shallow and deep fracture zones following treatment. In the shallow fracture zone, the calculated DNAPL mass removed was approximately equal to the DNAPL mass estimated using partitioning tracer testing, and no rebound in chlorinated ethenes or ethene was observed during the posttreatment period. A maximum DNAPL dissolution enhancement factor of 5 was observed in the shallow fracture zone. In the deep fracture zone, only approximately 45% of the DNAPL mass—as estimated via partitioning tracer testing—was removed and rebound in the total molar chlorinated ethenes + ethene was observed. The difference in behavior between the shallow and deep fracture zones was attributed to DNAPL architecture and the fracture flow field.     

From Lightning Rod to Community Asset? The Creation of a Protected Area in Northern California and its Evolving Relationship to a Nearby Community

Researchers have described shifting relationships between communities and Protected Areas (PAs), from excluding communities to consultative relationships to participatory models. This research focuses on a PA created on formerly industrial timberland that was the focus of conflict between environmental activists and the timber industry. In a nearby formerly timber-dependent community, we assessed community use and support of the PA, and its perceived purposes and benefits, through a survey of residents and interviews with key stakeholders. Despite intense conflict surrounding its creation, we found support for the PA, and our case suggests a new possible relationship between PAs and communities, which is mitigating and helping to resolve natural resource conflict. We also found that community well-being benefits of the PA were unclear; we suggest that PA management should include cultural, economic, and historical context in objectives and interpretive materials; and should work to create economic opportunities connected to the PA.     

Relationships between phytoplankton standing crop and physical, chemical, and biological gradients in the Suwannee River and plume region, U.S.A.

The Suwannee River (USA) is an amber stained, nutrient rich, blackwater river which flows into relatively clear oceanic waters resulting in the formation of a coastal region with unique physical, chemical, and biological gradients. The intent of this study was to describe the spatial and temporal variability of phytoplankton as it relates to these gradients. Ten stations along a transect ranging from 5 km up river to 31 km offshore, were sampled during four different flow regimes. All four sampling periods included in our study of the Suwannee River and plume region exhibited a similar pattern of phytoplankton abundance; low phytoplankton biomass in the Suwannee River and offshore stations with an area of elevated biomass seaward of the Suwannee River outflow. The results of our analysis of light and nutrient limitation in the region support the hypothesis that this spatial pattern of phytoplankton abundance is strongly influenced by color dependent light limitation in the river and outflow area, combined with nutrient limitation offshore. Our results suggest that both light and nutrient availability control abundance and composition of phyto plankton in this coastal area.     

Rapid movement of wastewater from on-site disposal systems into surface waters in the Lower Florida Keys

Viral tracer studies have been used previously to study the potential for wastewater contamination of surface marine waters in the Upper and Middle Florida Keys. Two bacteriophages, the marine bacteriophage ϕHSIC and theSalmonella phage PRD1, were used as tracers in injection well and septic tank studies in Saddlebunch Keys of the Lower Florida Keys and in septic tank studies in Boot Key Harbor, Marathon, of the Middle Keys. In Boot Key Harbor, both phages were detected in a canal adjacent to the seeded septic tank within 3 h 15 min of the end of the seed period. The tracer was then detected at all sampling sites in Boot Key Harbor, including one on the opposite side of U.S. Highway 1 in Florida Bay, and at an Atlantic Ocean beach outside Boot Key Harbor. Rates of migration based on first appearance of the phage ranged from 1.7 to 57.5 m h⁻¹. In Saddlebunch Keys, ϕHSIC and PRD1 were used to seed a residential septic tank and a commercial injection well. The septic tank tracer was not found in any surface water samples. The injection well tracer was first detected at a site most distant from the seed site, a channel that connected Sugarloaf Sound with the Atlantic Ocean. The, rate of tracer migration from the injection well to this channel ranged from 66.8 to 141 m h⁻¹. Both tracer studies showed a rapid movement of wastewater from on-site sewage treatment and disposal systems in a southeasterly direction toward the reef tract and Atlantic Ocean, with preferential movement through tidal channels. These studies indicate that wastewater disposal systems currently in widespread use in the Florida Keys can rapidly contaminate the marine environment.