Integral Pumping Tests Research Articles

Natural attenuation research at the contaminated megasite Zeitz

Monitored natural attenuation (MNA) as a possible remediation strategy has been intensively discussed over the last several years. However, in order to apply natural attenuation (NA) as a rational remediation option, suitable methods for the assessment and evaluation of the efficiency of these processes must be developed, the sustainability of the NA processes has to be demonstrated and regulatory and public acceptance has to be established. The RETZINA (Reference test site Zeitz for the implementation of the “Natural Attenuation” approach) -Project deals with the implementation of the MNA approach as a remediation concept at the heavily contaminated former industrial field site Zeitz (Germany). Due to the long history of industrial production, the multiple contaminant sources present and the large spatial extent, Zeitz is considered a contaminated megasite. The overall goal of the project is to incorporate novel site investigation methods into conventional strategies to improve site characterisation with respect to remediation measures relying on NA. Several investigation methods are being tested to examine temporal and spatial aspects of NA. In addition to conventional site investigation strategies, incorporation of new mass flux, toxicological and isotopic approaches form part of the research. Integral pumping tests yielded reliable data on mass fluxes through a control plane. The toxicological analysis gave new insights into contaminant interactions and the issue of multiple sources. Investigations of isotopic fractionation yielded increased confidence that NA is indeed ongoing at the site. Thus, the new methods gave more insight into the complexity of contaminant load and occurring degradation processes. Taken together, they strengthened the reliance on NA as a remediation option for this particular contamination at the megasite in context of the protection of water resources at the regional scale.

Analysis of groundwater contamination using concentration–time series recorded during an integral pumping test: Bias introduced by strong concentration gradients within the plume

When only few monitoring wells are available to assess the extent and level of groundwater contamination, inversion of concentration breakthrough curves acquired during an integral pumping test can be used as an alternative quantification method. The idea is to use concentration–time series recorded during integral pumping tests through an inversion technique to estimate contaminant mass fluxes crossing a control plane. In this paper, we examine how a longitudinal concentration gradient along a contaminant plume length scale affects the estimated inversed-concentration distribution and its associated mass flux. The analytically inversed-concentration distribution at the imaginary control plane (ICP) is compared to a numerically generated concentration distribution, treating the latter one as a “real contaminant plume” characterized by the presence of a longitudinal concentration gradient. It is found that the analytically inversed-concentration can lead to overestimation or underestimation of concentration distribution values depending on the transport time period and dispersivity values. At lower dispersivity values, with shorter transport time periods, the analytically inversed-concentration distribution overestimates the “real” concentration distribution. A better fit of the estimated concentration distribution to the “real” one is observed when the transport time period increases, i.e. when the advective front has already crossed the ICP. However, for higher dispersivity values, underestimation of the real concentration distribution is observed. Deviation of the inversed-concentration distribution from the “real” one is assessed for a site-specific concentration gradient term. A concentration gradient adjusted contaminant mass flux is thus formulated to evaluate groundwater contamination levels at a given time period through an ICP. This concentration gradient ratio can indicate whether the ICP is well positioned to evaluate accurately contaminant mass fluxes which are representative of groundwater contamination levels.

Monitoring groundwater contamination and delineating source zones at industrial sites: Uncertainty analyses using integral pumping tests

Field-scale characterisations of contaminant plumes in groundwater, as well as source zone delineations, are associated with uncertainties that can be considerable. A major source of uncertainty in environmental datasets is due to variability of sampling results, as a direct consequence of the heterogeneity of environmental matrices. We develop a methodology for quantifying uncertainties in field-scale mass flow and average concentration estimations, using integral pumping tests (IPTs), where the contaminant concentration is measured as a function of time in a pumping well. This procedure increases the sampling volume and reduces the effect of small-scale variability that may bias point-scale measurements. In particular, using IPTs, the interpolation uncertainty of conventional point-scale measurements is transformed to a quantifiable uncertainty related to the (unknown) plume position relative to the pumping well. We show that this plume position uncertainty generally influenced the predicted mass flows and average concentrations (of acenapthene, benzene and CHCs) to a greater extent than a boundary condition uncertainty related to the local water balance, considering 19 control planes at a highly heterogeneous industrial site in southwest Germany. Furthermore, large (order of magnitude) uncertainties only occurred if the conditions were strongly heterogeneous in the nearest vicinity of the well. We also develop a consistent methodology for an assessment of the combined effect of uncertainty in hydraulic conditions and uncertainty in reactive transport parameters for delimiting of both contaminant source zones and zones absent of source, based on (downgradient) IPTs.

Quantification of groundwater contamination in an urban area using integral pumping tests

In this paper, the integral groundwater investigation method is used for the quantification of PCE and TCE mass flow rates at an industrialized urban area in Linz, Austria. In this approach, pumping wells positioned along control planes perpendicular to the groundwater flow direction are operated for a time period on the order of days and sampled for contaminants. The concentration time series of the contaminants measured during operation of the pumping wells are then used to determine contaminant mass flow rates, mean concentrations and the plume shapes and positions at the control planes. The three control planes used in Linz were positioned downstream of a number of potential source zones, which are distributed over the field site. By use of the integral investigation method, it was possible to identify active contaminant sources, quantify the individual source strength in terms of mass flow rates at the control planes and estimate the contaminant plume position relative to the control planes. The source zones emitting the highest PCE and TCE mass flow rates could be determined, representing the areas where additional investigation and remediation activities will be needed. Additionally, large parts of the area investigated could be excluded from further investigation and remediation activities.

Average contaminant concentration and mass flow in aquifers from time‐dependent pumping well data: Analytical framework

Conventional water samples are representative for small subsurface volumes relative to the scale of many natural (geological) heterogeneities. We present an analytical framework for estimation of representative field‐scale average concentrations and mass flows on the basis of much larger sampling volumes that are obtained through so‐called integral pumping tests. The contaminant concentration is then measured as a function of time in a pumping well and used for estimation of the conditions in the aquifer prior to (and after) pumping, increasing the observation scale to the size of the well capture zone. This method complements (and provides an alternative to) conventional monitoring grids, where mass flow and concentration may be misinterpreted or plumes even missed because of problems related to grid spacing. The (not measured) initial spatial concentration distribution and the time‐dependent concentration measured at the well are related through a Volterra integral equation of the first kind. For limiting cases of short and long dimensionless pumping duration, two closed form analytical solutions are given, from which the mass flow and average concentration can be evaluated. Furthermore, a new solution for evaluating integral pumping tests of any duration is provided and used for investigating the applicability of the simple, analytical closed form solutions for interpreting test results from seven large‐scale contaminated sites in Europe.

Assessing microbial degradation of o-xylene at field-scale from the reduction in mass flow rate combined with compound-specific isotope analyses

In recent years, natural attenuation (NA) has evolved into a possible remediation alternative, especially in the case of BTEX spills. In order to be approved by the regulators, biodegradation needs to be demonstrated which requires efficient site investigation and monitoring tools. Three methods—the Integral Groundwater Investigation method, the compound-specific isotope analysis (CSIA) and a newly developed combination of both—were used in this work to quantify at field scale the biodegradation of o-xylene at a former gasworks site which is heavily contaminated with BTEX and PAHs. First, the Integral Groundwater Investigation method [Schwarz, R., Ptak, T., Holder, T., Teutsch, G., 1998. Groundwater risk assessment at contaminated sites: a new investigation approach. In: Herbert, M. and Kovar, K. (Editors), GQ'98 Groundwater Quality: Remediation and Protection. IAHS Publication 250, pp. 68–71; COH 4 (2000) 170] was applied, which allows the determination of mass flow rates of o-xylene by integral pumping tests. Concentration time series obtained during pumping at two wells were used to calculate inversely contaminant mass flow rates at the two control planes that are defined by the diameter of the maximum isochrone. A reactive transport model was used within a Monte Carlo approach to identify biodegradation as the dominant process for reduction in the contaminant mass flow rate between the two consecutive control planes. Secondly, compound-specific carbon isotope analyses of o-xylene were performed on the basis of point-scale samples from the same two wells. The Rayleigh equation was used to quantify the degree of biodegradation that occurred between the wells. Thirdly, a combination of the Integral Groundwater Investigation method and the compound-specific isotope analysis was developed and applied. It comprises isotope measurements during the integral pumping tests and the evaluation of δ 13C time series by an inversion algorithm to obtain spatially integrated mean isotope values at the control planes. It was shown that the Rayleigh equation is applicable to spatially integrated mean isotope values in order to obtain the mean biodegradation between the consecutive control planes. All three approaches yielded consistently a 98–99% degradation of o-xylene.