DOI 10.1007/s10040-0010176-2 Abstract Various techniques are available to quantify recharge; however, choosing appropriate techniques is often difficult. Important considerations in choosing a technique include space/time scales, range, and reliabili- ty of recharge estimates based on different techniques; other factors may limit the application of particular tech- niques. The goal of the recharge study is important be- cause it may dictate the required space/time scales of the recharge estimates. Typical study goals include water-re- source evaluation, which requires information on re- charge over large spatial scales and on decadal time scales; and evaluation of aquifer vulnerability to contam- ination, which requires detailed information on spatial variability and preferential flow. The range of recharge rates that can be estimated using different approaches should be matched to expected recharge rates at a site. The reliability of recharge estimates using different tech- niques is variable. Techniques based on surface-water and unsaturated-zone data provide estimates of potential recharge, whereas those based on groundwater data gen- erally provide estimates of actual recharge. Uncertainties in each approach to estimating recharge underscore the need for application of multiple techniques to increase

http://images.water.nv.gov/images/Misc/spring%20valley%20hearings/SNWA/568.pdf

Choosing appropriate techniques for quantifying groundwater recharge

Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg Nkm � 2 � yr � 1 ) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.

/pdf/denitrification-across-landscapes-and-waterscapes-a-ei9e1947rb.pdf

Denitrification across landscapes and waterscapes: a synthesis.

Emerging organic contaminants in groundwater: A review of sources, fate and occurrence

Publisher Summary This chapter focuses on the uses of isotopes to understand water chemistry.I Isotopic compositions generally cannot be interpreted successfully in the absence of other chemical and hydrologic data. The chapter focusses on uses of isotopes in tracing sources and cycling of nitrogen in the water-component of forested catchment, and on dissolved nitrate in shallow waters, nutrient uptake studies in agricultural areas, large-scale tracer experiments, groundwater contamination studies, food-web investigations, and uses of compound-specific stable isotope techniques. Shallow waters moving along a flowpath through a relatively uniform material and reacting with minerals probably do not achieve equilibrium but gradually approach some steady-state composition. The chapter also discusses the use of isotopic techniques to assess impacts of changes in land-management practices and land use on water quality. The analysis of individual molecular components for isotopic composition has much potential as a method for tracing the source, biogeochemistry, and degradation of organic liquids and gases because different materials have characteristic isotope spectrums or biomarkers.

Tracing nitrogen sources and cycling in catchments

Nitrate attenuation in groundwater: a review of biogeochemical controlling processes.

This study explores the use of multiple isotopic tracers to evaluate the processes involved in nitrate attenuation in ground water. δ15N and δ18O are used to provide information about the role of denitrification on nitrate attenuation, and δ34S, δ18O, and δ13C are used to evaluate the role of reduced sulfur and carbon as electron donors for nitrate reduction. The focus of this study is a zone of significant NO3−1 attenuation occurring in a sand aquifer impacted by septic system contamination. The NO3−1 pattern, the ground water flow system, and changes in other chemical parameters suggest that the NO3−1 depletion is caused by denitrification. This is supported by the nitrate δ15N and δ18O data which both show significant isotopic enrichment as NO3−1 depletion proceeds along the flow path. The increase of sulfate and dissolved inorganic carbon observed in the zone of nitrate attenuation suggests that reduced sulfur in addition to carbon is also involved in denitrification. This is supported by a trend toward depleted sulfate δ34S and δ18O values in the zone of sulfate increase, which reflects the input of sulfate formed by the oxidation of biogenic pyrite present in the aquifer sediments. The trend toward depleted δ13 values in the zone of increasing dissolved inorganic carbon reflects the input of organic carbon into this carbon pool. Chemical mass balance indicates that carbon is the dominant electron donor; however, this study demonstrates the effectiveness of using multiple isotopic tracers for providing insight into the processes affecting nitrate attenuation in ground water.

Use of Multiple Isotope Tracers to Evaluate Denitrification in Ground Water: Study of Nitrate from a Large-Flux Septic System Plume

Denitrifying bioreactors - an approach for reducing nitrate loads to receiving waters.

Distinct plumes of septic system-impacted ground water at two single-family homes located on shallow unconfined sand aquifers in Ontario showed elevated levels of Cl−, NO3−, Na+, Ca2+, K+, alkalinity, and dissolved organic carbon and depressed levels of pH and dissolved oxygen. At the Cambridge site, in use 12 years, the plume had sharp lateral and vertical boundaries and was more than 130 m in length with a uniform width of about 10 m. As a result of low transverse dispersion in the aquifer, mobile plume solutes such as NO3− and Na+ occurred at more than 50 percent of the source concentrations 130 m downgradient from the septic system. At the Muskoka site, in use three years, the plume also had discrete boundaries reflecting low transverse dispersion. After 1.5 years of system operation, the Muskoka plume began discharging to a river located 20 m from the tile field. Almost complete NOs attenuation was observed within the last 2 m of the plume flowpath before discharge to the river. This was attributed to denitrification occurring within organic matter-enriched riverbed sediments.

The very weakly dispersive nature of the two aquifers was consistent with the results of recently reported natural-gradient tracer tests in sands. Therefore, for many unconfined sand aquifers, the minimum distance-to-well regulations for permitting septic systems in most parts of North America should not be expected to be adequately protective of well-water quality in situations where mobile contaminants such as NOs are not attenuated by chemical or microbiological processes.

Ground‐Water Contamination from Two Small Septic Systems on Sand Aquifers

A new alternative septic-system design is presented utilizing reactive porous media barriers for passive in situ attenuation of NO3−. The reactive material consists of solid organic carbon (sawdust) which promotes NO3- attenuation by heterotrophic denitrification. Four field trials are discussed demonstrating two barrier configurations: as a horizontal layer positioned in the vadose zone below a conventional septic-system infiltration bed and as a vertical wall intercepting a horizontally flowing downgradient plume. During one year of operation both barrier configurations have been successful in substantial attenuation (60 to 100%) of input NO3- levels of up to 125 mg/1 as N. The horizontal layer configuration can be readily installed during the construction of new infiltration beds, whereas the vertical wall configuration may be more appropriate for retrofitting existing septic systems where NO3- contamination has already occurred. The layer configuration allows the flexibility of constructing the barrier in the vadose zone by using coarse silt or fine sand matrix material that has the ability to remain tension-saturated, and thus anaerobic, even when positioned above the water table.

Advantages of the barrier system are that it is simple to construct, no surface structures or additional plumbing are necessary, and treatment is passive requiring no energy consumption and little or no maintenance. Mass balance calculations and preliminary results suggest that conveniently sized barriers have the potential to last for decades without replenishment of the reactive material.

In Situ Denitrification of Septic‐System Nitrate Using Reactive Porous Media Barriers: Field Trials

Nitrate is now recognized as a widespread ground water contaminant, which has led to increased efforts to control and mitigate its impacts. This study reports on the long-term performance of four pilot-scale field trials in which reactive porous barriers were used to provide passive in situ treatment of nitrate in ground water. At two of the sites (Killarney and Borden), the reactive barriers were installed as horizontal layers underneath septic system infiltration beds; at a third site (Long Point), a barrier was installed as a vertical wall intercepting a horizontally migrating septic system plume; and at the fourth site (North Campus), a barrier was installed as a containerized subsurface reactor treating farm field drainage water. The reactive media consisted of 15% to 100% by volume of waste cellulose solids (wood mulch, sawdust, leaf compost), which provided a carbon source for heterotrophic denitrification. The field trials have been in semicontinuous operation for six to seven years at hydraulic loading rates ranging from six to 2000 L/day. Trials have been successful in attenuating influent NO3- (or NO3-+ NH4+ at Borden) concentrations averaging from 4.8 mg/L N at North Campus to 57 mg/L N at Killarney, by amounts averaging 80% at Killarney, 74% at Borden, 91 % at Long Point, and 58% at North Campus. Nitrate consumption rates were temperature dependent and ranged from 0.7 to 32 mg L N/day, but did not deteriorate over the monitoring period. Furthermore, mass-balance calculations indicate that carbon consumption by heterotrophic denitrification has so far used only about 2% to 3% of the initial carbon mass in each case. Results suggest that such barriers should be capable of providing NO3- treatment for at least a decade or longer without carbon replenishment.

Reactive barriers have now been used to treat nitrate contamination from a variety of sources including septic systems, agricultural runoff, landfill leachate, and industrial operations. This demonstration of successful long-term operation should allow this technology to become more widely considered for nitrate remediation, particularly at sites where passive treatment requiring a minimum of maintenance is desired.

William D. Robertson

Papers

Use of Multiple Isotope Tracers to Evaluate Denitrification in Ground Water: Study of Nitrate from a Large-Flux Septic System Plume

Denitrifying bioreactors - an approach for reducing nitrate loads to receiving waters.

Ground‐Water Contamination from Two Small Septic Systems on Sand Aquifers

In Situ Denitrification of Septic‐System Nitrate Using Reactive Porous Media Barriers: Field Trials

Long‐Term Performance of In Situ Reactive Barriers for Nitrate Remediation