Marti Bayer-Raich

Investigation of sewer exfiltration using integral pumping tests and wastewater indicators.

Leaky sewers affect urban groundwater by the exfiltration of untreated wastewater. However, the impact of sewer exfiltration on the groundwater is poorly understood. Most studies on sewer exfiltration focus on water exfiltration, but not on the impact on groundwater quality. In this paper we present a new monitoring approach to estimate mass flow rates M(ex) of different wastewater indicators (WWIs) from leaky sewers by applying integral pumping tests (IPTs). The problem of detecting and assessing heterogeneous concentrations in the vicinity of leaky sewers can be overcome with the IPT approach by the investigation of large groundwater volumes up- and downstream of leaky sewers. The increase in concentrations downstream of a leaky sewer section can be used to calculate M(ex) with a numerical groundwater model. The new monitoring approach was first applied using four IPT wells in Leipzig (Germany). Over a pumping period of five days we sampled five inorganic WWIs: B , Cl(-), K+, NO3(-), NH4+ and three xenobiotics: bisphenol-a, caffeine and tonalide. The resulting concentration-time series indicated an influence of wastewater at one IPT well downstream of the leaky sewer. We defined ranges of M(ex) by implementing the uncertainty of chemical analyses. The results showed a M(ex) of 0-10.9 g m(-1) d(-1). The combination of M(ex) with wastewater concentrations from the target sewer yielded an exfiltration rate Q(ex) of 28.0-63.9 Lm(-1)d(-1) for the conservative ion Cl(-). Most non-conservative WWIs showed reduced mass flow rates in the groundwater downstream of the leaky sewer that indicate a mass depletion during their passage from the sewer to the pumping well. Application of the IPT methodology at other field sites is possible. The IPT monitoring approach provides reliable M(ex) values that can help to assess the impact of leaky sewers on groundwater.

Integral quantification of contaminant mass flow rates in a contaminated aquifer: Conditioning of the numerical inversion of concentration-time series

A series of integral pumping tests (IPTs) has been conducted at a former gasworks site to quantify the contaminant mass flow rates and average concentration in groundwater along three control planes across the groundwater flow direction. The measured concentration-time series were analysed numerically with the help of the inversion code CSTREAM and a flow and transport model representing the highly heterogeneous aquifer. Since the control planes cover the entire downstream width of the potentially contaminated area, they allow conclusions to be drawn about the current location and spread of the contaminant plume. Previous evaluations of integral pumping tests could calculate three scenarios concerning the spread of the plume around the IPT well: (i) the plume is located to the right of the pumping well, (ii) to the left, or (iii) is distributed symmetrically around it. To create a more realistic picture of the plume position, a series of direct-push monitoring wells were installed along one control plane. The concentrations found in these wells were included in the numerical analysis to condition the numerical inversion results, and allowed the identification of a more pronounced plume centre and fringe, which supports the development of optimised remediation strategies.

Towards optimal sampling schedules for integral pumping tests

Conventional point sampling may miss plumes in groundwater due to an insufficient density of sampling locations. The integral pumping test (IPT) method overcomes this problem by increasing the sampled volume. One or more wells are pumped for a long duration (several days) and samples are taken during pumping. The obtained concentration-time series are used for the estimation of average aquifer concentrations C(av) and mass flow rates M(CP). Although the IPT method is a well accepted approach for the characterization of contaminated sites, no substantiated guideline for the design of IPT sampling schedules (optimal number of samples and optimal sampling times) is available. This study provides a first step towards optimal IPT sampling schedules by a detailed investigation of 30 high-frequency concentration-time series. Different sampling schedules were tested by modifying the original concentration-time series. The results reveal that the relative error in the C(av) estimation increases with a reduced number of samples and higher variability of the investigated concentration-time series. Maximum errors of up to 22% were observed for sampling schedules with the lowest number of samples of three. The sampling scheme that relies on constant time intervals ∆t between different samples yielded the lowest errors.

Correction to “Average contaminant concentration and mass flow in aquifers from time-dependent pumping well data: Analytical framework” by Martí Bayer-Raich et al.

[1] In the paper ‘‘Average contaminant concentration and mass flow in aquifers from time-dependent pumping well data: Analytical framework’’ by M. Bayer-Raich, Jerker Jarsjo, Rudolf Liedl, Thomas Ptak, and Georg Teutsch (Water Resources Research, 40, W08303, doi:10.1029/ 2004WR003095, 2004), equation (18) was printed incorrectly: 1/2 is missing in front of the integral sign. The results and derivations presented in the paper are unaffected by this error since it is of a typographical nature; however, readers intending to use the equation in their analyses should note that the correct equation is as follows:

Estimates of Horizontal Groundwater Flow Velocities in Boreholes: Estimates of Horizontal Groundwater Flow Velocities in Boreholes

Currently, monitoring tools can be deployed in observation boreholes to better assess groundwater flow, flux of dissolved contaminants and their mass discharge in an aquifer. The relationship between horizontal water velocity in observation boreholes and Darcy fluxes in the surrounding aquifer has been studied for natural flow conditions (i.e., no pumping). Interpretation of measurements taken with dilution tests, the colloidal borescope, the Heat Pulse Flowmeter, and other techniques require the conversion of observed borehole velocity u to aquifer Darcy flux q∞ . This conversion is typically done through a proportionality factor α = u/q∞ . In experimental studies as well as in theoretical developments, reported values of α vary almost three orders of magnitude (from 0.5 to 10). This large variability in reported values of α could be explained by: (1) unclear distinction between Darcy flux and water seepage velocity, (2) unclear definition of water velocity in the borehole, (3) effects of well screen and the presence of the measurement device itself on the observable velocities, and (4) hydraulic conditions in the borehole annulus. We address (1), (2) from a conceptual/theoretical perspective, and (3) by means of numerical simulations. We show that issue (1) in low porosity aquifers can yield to order-of-magnitude discrepancies in estimates of q∞ ; (2) may result in discrepancies of up to 50%, and (3) can cause differences up to 20% of water velocity in the borehole void space compared to the theoretical case of an open borehole.