Hans Peter Broers

Dating degassed groundwater with 3H/3He

The production of gases in groundwater under contaminated locations by geochemical and biological processes is not uncommon. Degassing of these gases from groundwater and repartitioning of noble gases between water and gas phase distorts groundwater dating by 3H/3He. We observed noble gas concentrations below atmospheric equilibrium in 20 out of 34 groundwater samples from agriculturally polluted sandy areas in the Netherlands. From the absence of nitrate in degassed samples, we conclude that denitrification causes degassing. The 22Ne/20Ne ratios show that degassing had attained solubility equilibrium and had not caused isotopic fractionation by diffusion. To correct for the loss of tritiogenic 3He due to degassing, we present a single-step equilibrium degassing model. We use the total dissolved gas pressure at the monitoring screen to estimate the depth and timing of degassing, which is essential to estimate travel times from degassed samples. By propagating the uncertainties in the underlying measurements and assumptions through the travel time calculations, we found a travel time uncertainty of 3 years (a). We therefore conclude that 3H/ 3He dating can produce valuable information on groundwater flow even at sites with strong degassing. Copyright 2007 by the American Geophysical Union.

Comparison of methods for the detection and extrapolation of trends in groundwater quality.

Land use changes and the intensification of agriculture since the 1950s have resulted in a deterioration of groundwater quality in many European countries. For the protection of groundwater quality, it is necessary to (1) assess the current groundwater quality status, (2) detect changes or trends in groundwater quality, (3) assess the threat of deterioration and (4) predict future changes in groundwater quality. A variety of approaches and tools can be used to detect and extrapolate trends in groundwater quality, ranging from simple linear statistics to distributed 3D groundwater contaminant transport models. In this paper we report on a comparison of four methods for the detection and extrapolation of trends in groundwater quality: (1) statistical methods, (2) groundwater dating, (3) transfer functions, and (4) deterministic modeling. Our work shows that the selection of the method should firstly be made on the basis of the specific goals of the study (only trend detection or also extrapolation), the system under study, and the available resources. For trend detection in groundwater quality in relation to diffuse agricultural contamination, a very important aspect is whether the nature of the monitoring network and groundwater body allows the collection of samples with a distinct age or produces samples with a mixture of young and old groundwater. We conclude that there is no single optimal method to detect trends in groundwater quality across widely differing catchments.

Degassing of 3H/3He, CFCs and SF6 by denitrification: measurements and two-phase transport simulations.

The production of N2 gas by denitrification may lead to the appearance of a gas phase below the water table prohibiting the conservative transport of tracer gases required for groundwater dating. We used a two-phase flow and transport model (STOMP) to study the reliability of 3H/3He, CFCs and SF6 as groundwater age tracers under agricultural land where denitrification causes degassing. We were able to reproduce the amount of degassing (R2=69%), as well as the 3H (R2=79%) and 3He (R2=76%) concentrations observed in a 3H/3He data set using simple 2D models. We found that the TDG correction of the 3H/3He age overestimated the control 3He/3He age by 2.1 years, due to the accumulation of 3He in the gas phase. The total uncertainty of degassed 3H/3He ages of 6 years (+/-2 sigma) is due to the correction of degassed 3He using the TDG method, but also due to the travel time in the unsaturated zone and the diffusion of bomb peak 3He. CFCs appear to be subject to significant degradation in anoxic groundwater and SF6 is highly susceptible to degassing. We conclude that 3H/3He is the most reliable method to date degassed groundwater and that two-phase flow models such as STOMP are useful tools to assist in the interpretation of degassed groundwater age tracer data.

Travel Time Distributions Derived from Particle Tracking in Models Containing Weak Sinks

A travel time distribution based on a particle-tracking analysis in a ground water model containing weak sinks is often uncertain because whether a particle is discharged or allowed to pass through a weak sink is unresolved by particle-tracking theory. We present a probability-based method to derive an objective travel time distribution in models containing weak sinks. The method discharges a fraction of the particle at the weak sink and allows the remaining fraction to pass through the weak sink. The weight of the discharged fraction depends on the ratio of the sink flux to the influx into the weak sink cell. We tested this approach on a coarse (100 x 100 m) and a fine (25 x 25 m) horizontal resolution regional scale ground water model (34.5 x 24 km). We compared the travel time distributions in a small subcatchment derived from particle-tracking analysis with one derived from a transport model. We found that the particle-tracking analysis with the coarse model underestimated the travel time distribution of the catchment compared to the transport solution or a particle-tracking analysis with the fine model. The underestimation of travel times with the coarse model was a result of a large area covered by sink cells in this model and the more accurate flow patterns simulated by the fine model. The probability-based method presented here compares favorably with a solute transport solution and provides an accurate travel time distribution when used with a fine-resolution ground water model.

Model-based integration and analysis of biogeochemical and isotopic dynamics in a nitrate-polluted pyritic aquifer

Leaching of nitrate from agricultural land to groundwater and the resulting nitrate pollution are a major environmental problem worldwide. Its impact is often mitigated in aquifers hosting sufficiently reactive reductants that can promote autotrophic denitrification. In the case of pyrite acting as reductant, however, denitrification is associated with the release of sulfate and often also with the mobilization of trace metals (e.g., arsenic). In this study, reactive transport modeling was used to reconstruct, quantify and analyze the dynamics of the dominant biogeochemical processes in a nitrate-polluted pyrite-containing aquifer and its evolution over the last 50 years in response to changing agricultural practices. Model simulations were constrained by measured concentration depth profiles. Measured (3)H/(3)He profiles were used to support the calibration of flow and conservative transport processes, while the comparison of simulated and measured sulfur isotope signatures acted as additional calibration constraint for the reactive processes affecting sulfur cycling. The model illustrates that denitrification largely prevented an elevated discharge of nitrate to surface waters, while sulfate discharges were significantly increased, peaking around 15 years after the maximum nitrogen inputs.

Chapter 10.2:Modelling Reactive Transport of Diffuse Contaminants: Identifying the Groundwater Contribution to Surface Water Quality

The European Union (EU) Water Framework Directive1 stimulates an integrated approach of the whole soil–groundwater–surface water system. Transport models are potentially very useful to improve insight into the dominant processes which control present and future water quality and to quantitatively ev...

Presence and fate of veterinary antibiotics in age-dated groundwater in areas with intensive livestock farming

The combination of emerging antibiotic resistance and lack of discovery of new antibiotic classes poses a threat to future human welfare. Antibiotics are administered to livestock at a large scale and these may enter the environment by the spreading of manure on agricultural fields. They may leach to groundwater, especially in the Netherlands which has some of the most intensive livestock farming and corresponding excessive manure spreading in the world. This study investigates the presence of antibiotics in groundwater in two regions with the most intensive livestock farming in the Netherlands. If so, the hydrochemical conditions were further elaborated. Ten multi-level wells with in total 46 filters were sampled, focusing on relatively young, previously age-dated groundwater below agricultural fields. Twenty-two antibiotics were analyzed belonging to the following antibiotic groups: tetracyclines, sulfonamides, trimethoprims, β-lactams, macrolides, lincosamides, quinolones, nitrofurans and chloramphenicol. The samples were analyzed for these antibiotics by LC-MS/MS ESI-POS/NEG (MRM) preceded by solid phase extraction which resulted in importantly low detection limits. Six antibiotics were found above detection limits in 31 filters in seven wells: sulfamethazine, sulfamethoxazole, lincomycin, chloramphenicol, ciprofloxacin, and sulfadiazine. The concentrations range from 0.3 to 18 ng L-1. Sulfonamides were detected at all measured depths down to 23 meters below surface level with apparent groundwater ages up to 40 years old. No antibiotics were detected below the nitrate/iron redox cline, which suggests that the antibiotics might undergo degradation or attenuation under nitrate-reducing redox conditions. This study provides proof that antibiotics are present in groundwater below agricultural areas in the Netherlands due to the spreading of animal manure.