The influence of heterogeneous groundwater discharge on the timescales of contaminant mass flux from streambed sediments ? field evidence and long-term predictions

International audienceStreambed sediments can act as long-term storage zones for organic contaminants originating from the stream water. Until the early 1990s, the small man-made stream, subject of our study, in the industrial area of Bitterfeld (Germany), was used for waste water discharge from the chemical industry nearby. The occurrence of contaminants in the streambed is resulting from aqueous-phase transport and particle facilitated deposition. Groundwater discharge through the streambed can otherwise induce a remobilization and an advective contaminant flux so that contaminants are released back from the streambed to the stream water. We investigated the long-term mass flow rates of chlorinated benzenes (MCB, DCBs) from the streambed to the overlying stream water driven by advection of groundwater. The spatial patterns and magnitudes of groundwater discharge were examined along a reach of 220 m length. At 140 locations groundwater discharge was quantified using streambed temperatures and ranged from 11.0 to 455.0 Lm?2d?1. According to locations with high and low groundwater discharge, time-integrating passive samplers were used to monitor current contaminant concentrations in the streambed. Streambed contaminant concentrations at high groundwater discharge locations were found to be lower than at low discharge locations. Based on data from batch experiments and field observations we parameterized and run multiple one-dimensional advective transport models for the observed range of groundwater discharge magnitudes to simulate the timescales of contaminant release and their dependence on the magnitude of groundwater discharge. The results of the long-term predictive modeling revealed that the time required to reduce the concentrations and the resulting mass fluxes to the water to 10% of the initial values will be in the scale of decades for high-discharge locations and centuries for low-discharge locations, respectively

Application of multi‑method approach to assess groundwater–surface water interactions, for catchment management

Globally, the dependence of river systems to delayed discharge of subsurface water to augment flows during dry seasons is well documented. Discharge of fresh subsurface water can dilute concentrated river flow quality during reduced flow. Observed and reported results on the Berg River’s declining water quantity and quality are a concern to the regions socio-economic growth and environmental integrity. Understanding the role of subsurface water discharges on the quantity and quality of receiving surface water courses can improve their management during dry periods. A case study was designed and implemented in the upper Berg River catchment in the Western Cape Province of South Africa to assess the influence of groundwater–surface water interaction on water quantity and quality. This study aimed to quantify and characterize the quality of subsurface water available in the upper catchment to improve observed declining water quality downstream. Hydrograph separation provided estimates of water fluxes during 2012–2014 low and high flow periods, while hydrochemical analysis provided insights on impacts of major land use activity in this catchment on water resources. Hydrograph separation analysis indicated that the Berg River is 37.9% dependent on subsurface water discharges annually. Dominant Na–Cl-type water indicates the quality of water from the upper Berg River is largely affected by natural processes including short residence times of aquifer water, rock–water interactions and atmospheric deposition of NaCl ions. These results provide insights for suggesting management options to be implemented to protect subsurface water for continued dilution and water resources management in the lower catchments

Measuring methods for groundwater, surface water and their interactions: a review

International audienceInteractions between groundwater and surface water play a critical role in the functioning of riparian ecosystems. In the context of sustainable river basin management it is crucial to understand and quantify exchange processes between groundwater and surface water. Numerous well-known methods exist for parameter estimation and process identification in aquifers and surface waters. The transition zone, however, has only in recent years become a subject of major research interest, and the need has evolved for appropriate methods applicable in this zone. This article provides an overview of the methods that are typically used in aquifers and surface waters when studying interactions and shows the possibilities of application in the transition zone. In addition, methods particularly for use in the transition zone are presented. Considerations for choosing appropriate methods are given including spatial and temporal scales, uncertainties, and limitations in application. It is concluded that a multi-scale approach combining multiple measuring methods may considerably constrain estimates of fluxes between groundwater and surface water

Measuring methods for groundwater – surface water interactions: a review

Interactions between groundwater and surface water play a fundamental role in the functioning of riparian ecosystems. In the context of sustainable river basin management it is crucial to understand and quantify exchange processes between groundwater and surface water. Numerous well-known methods exist for parameter estimation and process identification in aquifers and surface waters. Only in recent years has the transition zone become a subject of major research interest; thus, the need has evolved for appropriate methods applicable in this zone. This article provides an overview of the methods that are currently applied and described in the literature for estimating fluxes at the groundwater – surface water interface. Considerations for choosing appropriate methods are given including spatial and temporal scales, uncertainties, and limitations in application. It is concluded that a multi-scale approach combining multiple measuring methods may considerably constrain estimates of fluxes between groundwater and surface water

Influence of aquifer and streambed heterogeneity on the distribution of groundwater discharge

The spatial distribution of groundwater fluxes through a streambed can be highly variable, most often resulting from a heterogeneous distribution of aquifer and streambed permeabilities along the flow pathways. Using a groundwater flow and heat transport model, we defined four scenarios of aquifer and streambed permeability distributions to simulate and assess the impact of subsurface heterogeneity on the distribution of groundwater fluxes through the streambed: (a) a homogeneous low-<i>K</i> streambed within a heterogeneous aquifer; (b) a heterogeneous streambed within a homogeneous aquifer; (c) a well connected heterogeneous low-<i>K</i> streambed within a heterogeneous aquifer; and (d) a poorly connected heterogeneous low-<i>K</i> streambed within a heterogeneous aquifer. The simulation results were compared with a base case scenario, in which the streambed had the same properties as the aquifer, and with observed data. The results indicated that the aquifer has a stronger influence on the distribution of groundwater fluxes through the streambed than the streambed itself. However, a homogeneous low-<i>K</i> streambed, a case often implemented in regional-scale groundwater flow models, resulted in a strong homogenization of fluxes, which may have important implications for the estimation of peak mass flows. The flux distributions simulated with heterogeneous low-<i>K</i> streambeds were similar to the flux distributions of the base case scenario, despite the lower permeability. The representation of heterogeneous distributions of aquifer and streambed properties in the model has been proven to be beneficial for the accuracy of flow simulations