Numerical Modeling of Nitrate Removal in Anoxic Groundwater during River Flooding of Riparian Zones.

A numerical study demonstrates the effects of flooding on subsurface hydrological flowpaths and nitrate removal in anoxic groundwater in riparian zones with a top peat layer. A series of two-dimensional numerical simulations with changing conditions for flow (steady state or transient with flooding), hydrogeology, denitrification, and duration of flooding demonstrate how flowpaths, residence times, and nitrate removal are affected. In periods with no flooding groundwater flows horizontally and discharges to the river through the riverbed. During periods with flooding, shallow groundwater is forced upwards as discharge through peat layers that often have more optimal conditions for denitrification caused by the presence of highly reactive organic matter. The contrast in hydraulic conductivity between the sand aquifer and the overlying peat layer, as well as the flooding duration, have a significant role in determining the degree of nitrate removal.

Drivers and barriers for taking account of geological uncertainty in decision making for groundwater protection.

The geological conceptual model is considered a major source of uncertainty in groundwater modelling and well capture zone delineation. However, how to account for it in groundwater policy and management remains largely unresolved. We explore the drivers and barriers to account for geological conceptual uncertainty in groundwater protection amongst decision makers and stakeholders in an agricultural groundwater catchment in Denmark. Using a groundwater model, we analyze the impact of alternative geological conceptual models on capture zone delineation. A focus area, which covers multiple modelled capture zones, is defined and considered for groundwater protection. Model uncertainty and focus area are discussed at two workshops, one with local and national stakeholders and another with local farmers. The drivers to account for model uncertainty include: i) safer drinking water protection by considering a larger area for protection than identified from a single geological model; and ii) stability over time of management plans. The main barrier is the additional cost to the stakeholders for the protection of a larger area. We conclude that integration of geological uncertainty in groundwater protection plans may be improved through: i) better communication between the research community and the national water authority; ii) more constraining guidelines regarding the estimation of geological uncertainty; and iii) the development of a framework ensuring knowledge transfer to the local water authorities and detailing how to integrate uncertainty in management plans.

Long-term monitoring of streambed sedimentation and scour in a dynamic stream based on streambed temperature time series.

This study presented the monitoring and quantification of streambed sedimentation and scour in a stream with dynamically changing streambed based on measured phase and amplitude of the diurnal signal of sediment temperature time series. With the applied method, changes in streambed elevation were estimated on a sub-daily scale with 2-h intervals without continuous maintenance of the measurement system, thus making both high temporal resolution and long-term monitoring of streambed elevations possible. Estimates of streambed elevation showed that during base flow conditions streambed elevation fluctuates by 2-3 cm. Following high stream stages, scouring of 2-5 cm can be observed even at areas with low stream flow and weak currents. Our results demonstrate that weather variability can induce significant changes in the stream water and consequently sediment temperatures influencing the diurnal temperature signal in such an extent that the sediment thickness between paired temperature sensors were overestimated by up to 8 cm. These observations have significant consequences on the design of vertical sensor spacing in high-flux environments and in climates with reduced diurnal variations in air temperature.

Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and ³9Ar Transport Modeling.

A large-scale groundwater flow and transport model is developed for a deep-seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three-dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of (39)Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two-dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep-seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.

The Role of Groundwater for Lake-Water Quality and Quantification of N Seepage.

The heterogeneous nature of both groundwater discharge to a lake (inflow) and nitrate concentrations in groundwater can lead to significant errors in calculations of nutrient loading. Therefore, an integrated approach, combining groundwater flow and transport modelling with observed nitrate and ammonium groundwater concentrations, was used to estimate nitrate loading from a catchment via groundwater to an oligotrophic flow-through lake (Lake Hampen, Denmark). The transport model was calibrated against three vertical nitrate profiles from multi-level wells and 17 shallow wells bordering a crop field near the lake. Nitrate concentrations in groundwater discharging to the lake from the crop field were on average 70 times higher than in groundwater from forested areas. The crop field was responsible for 96% of the total nitrate loading (16.2 t NO3 /year) to the lake even though the field only covered 4.5% of the catchment area. Consequently, a small change in land use in the catchment will have a large effect on the lake nutrient balance and possible lake restoration. The study is the first known attempt to estimate the decrease of nitrate loading via groundwater to a seepage lake when an identified catchment source (a crop field) is removed.

Sand box experiments with bioclogging of porous media: hydraulic conductivity reductions.

Tracer experiments during clogging and de-clogging experiments in a 2D sand box were via an image analysis used to establish a data set on the relation between changes in hydraulic conductivity (K) and relative porosity (ß). Clogging appears to create a finger-like tracer transport, which could be caused by an initial heterogeneous distribution of biomass in the sand box. De-clogging occurs at a slower rate possibly due to the presence of inert biomass that is not affected by the starvation conditions by sudden removal of the substrate source. The tracer front was observed to get disturbed closer and closer to the substrate source during the experiments suggesting that the zone of clogging moved upstream. Three clogging models, K(ß), from the literature were tested for their ability to describe the temporal changes in clogging at the scale of the sand box; the model of Clement et al. (1996) that makes no assumption on biomass distribution, the plug formation model of Thullner et al. (2002a), and the biofilm-plug formation model of Vandevivere (1995). The plug formation and biofilm-plug formation models both match the observed changes between the hydraulic conductivity of the sand box and the relative porosity. Unfortunately our experiments did not reach low relative porosities where the two models predict different behaviors. The model by Clement et al. (1996) underestimates clogging.

The effects of the 2004 tsunami on a coastal aquifer in Sri Lanka.

On December 26, 2004, the earthquake off the southern coast of Sumatra in the Indian Ocean generated far-reaching tsunami waves, resulting in severe disruption of the coastal aquifers in many countries of the region. The objective of this study was to examine the impact of the tsunami on groundwater in coastal areas. Field investigations on the east coast of Sri Lanka were carried out along a transect located perpendicular to the coastline on a 2.4 km wide sand stretch bounded by the sea and a lagoon. Measurements of groundwater table elevation and electrical conductivity (EC) of the groundwater were carried out monthly from October 2005 to August 2007. The aquifer system and tsunami saltwater intrusion were modeled using the variable-density flow and solute transport code HST3D to understand the tsunami plume behavior and estimate the aquifer recovery time. EC values reduced as a result of the monsoonal rainfall following the tsunami with a decline in reduction rate during the dry season. The upper part of the saturated zone (down to 2.5 m) returned to freshwater conditions (EC < 1000 µS/cm) 1 to 1.5 years after the tsunami, according to field observations. On the basis of model simulations, it may take more than 15 years for the entire aquifer (down to 28 m) to recover completely, although the top 6 m of the aquifer may become fresh in about 5 years.

Use of tracer tests to investigate changes in flow and transport properties due to bioclogging of porous media.

Tracer tests were conducted in three laboratory columns to study changes in the hydraulic properties of a porous medium due to bioclogging. About 30 breakthrough curves (BTCs) for each column were obtained. The BTCs were analyzed using analytical equilibrium and dual-porosity models, and estimates of the hydrodynamic dispersion and mass transfer coefficients were obtained by curve fitting. The change in transport properties developed in three stages: an initial phase (I) with no significant changes in transport properties, phase II with growth of biomass near the inlet of the columns causing changes in dispersivity, and phase III with added growth of micro-colonies deeper in the columns causing mass transfer of solutes from the water phase to the biophase. Tracer transport changed from being uniform to more non-uniform with increase in mass transfer of the tracer between the mobile phase and the immobile biomass. An increase in the bulk dispersivity value of up to one order of magnitude was observed. Numerical simulations suggest that local dispersivity values may be as much as 40 times higher in the more severe clogged areas inside the column. The bulk hydraulic conductivities of the columns decreased by up to three orders of magnitude. The hydraulic conductivity and dispersivity parameters were almost recovered after disinfection of the columns. Different models relating the changes of the hydraulic conductivity to the changes in the mobile porosity due to bioclogging were reviewed, and the micro-colony relation of Thullner et al. [Thullner, M., Zeyer, J., Kinzelbach, W., 2002. Influence of microbial growth on hydraulic properties of pore networks, Transport in Porous Media, 49, 99-122.] was found to best describe the relation between the bulk hydraulic parameters.

A steady state redox zone approach for modeling the transport and degradation of xenobiotic organic compounds from a landfill site.

A redox zonation approach is used as a framework for obtaining biodegradation rate constants of xenobiotic compounds in a landfill plume (Grindsted, Denmark). The aquifer is physically heterogeneous in terms of a complex zonation of different geological units close to the landfill and biogeochemically heterogeneous in terms of a specified redox zonation. First-order degradation rates of six organic compounds (benzene, toluene, ethylbenzene, o-xylene, m/p-xylene, and naphthalene) were calculated in the methanogenic/sulfate- and Fe-reducing zones. The numerical simulations show that all compounds are anaerobically biodegraded, but at very different rates. High rates of biodegradation of most of the compounds (except benzene) were found in the Fe-reducing zone. These rates generally agree with previously published rates. Only o-xylene and toluene were significantly biodegraded in the methanogenic/sulfate-reducing environment. All rates in this redox zone are generally much lower than previously published rates.

Pesticide transport in an aerobic aquifer with variable pH--modeling of a field scale injection experiment.

Three-dimensional reactive transport simulations were undertaken to study the sorption and degradation dynamics of three herbicides in a shallow aerobic aquifer with spatially variable pH during a 216 days injection experiment. Sorption of two phenoxy acids [(+/-)-2-(4-chloro-2-methylphenoxy) propanoic acid] (MCPP) and [(+/-)-2-(2,4-dichlorophenoxy)propanoic acid] (dichlorprop) was found to be negligible. Degradation of the phenoxy acids was rapid after an initial lag phase. Degradation of the phenoxy acids could only be reproduced satisfactorily by growth-linked microbial degradation. The model fit to the field data was slightly improved if degradation was assumed to be influenced by the local pH that was observed to increase with depth ( approximately 4.5--5.7). In the observed pH-range the nitroaromatic herbicide [2-Methyl-4,6-dinitrophenol] (DNOC) was partly dissociated (pK(a)=4.31) and present in both the neutral and ionized form. The model simulations demonstrated that most of the observed spatial variation in sorption of DNOC could be explained by assuming that only the neutral form of DNOC was subject to sorption. A varying flow field was observed during the injection experiment and the model simulations documented that this most likely resulted in different migration paths for DNOC and the non-sorbing solutes. The model simulations indicated that degradation of DNOC was an important process. The degradation rate of DNOC remained constant over time and was simulated adequately by first-order kinetics. Again, the model fit to field observation was slightly improved if local pH was assumed to influence the degradation rate. Only the maximum utilization rate was estimated from the field data, while the remaining degradation parameters where successfully transferred from the laboratory study.