Transport parameters and contaminant data

Highlights
Determination of transport parameters Contaminant sampling techniques

Transport parameters Advective transport happens due to the groundwater flow. However, to describe the transport of a substance in a fractured limestone aquifer properly, additional parameters are required. Important transport parameters that influence the migration of a substance are diffusion coefficient of the substance in the limestone (often estimated as molecular diffusion coefficient times the tortuosity or porosity of the limestone) dispersivities (may be less important, if fracture flow dominates) sorption coefficient (organic carbon is usually low in limestone) limestone porosity degradation rates Tracer tests are very useful to analyze the transport behavior in a limestone aquifer. Different types of tracer tests can be distinguished: push-pull tracer tests, where a tracer is injected and monitored in the same borehole tracer tests with an injection well and one or several observation wells forced-gradient (with pumping) or natural gradient tracer tests In the limestone project, a forced-gradient tracer test with several injection wells and a central pumping well for tracer monitoring was conducted. Details are described in the following report: Report about the pumping and tracer test at Akacievej (PDF). Furthermore, measurements from core material can be used to determine the porosity and hydraulic conductivity of a limestone sample, for example using gas permeameter and porosimeter (Poroperm test). At the Akacievej site, this yielded porosity values between 7 adn 46 %, with a strong variation with depth. The determined hydraulic conductivity values were correlated to the porosity (and limestone hardness) and were in the range of between 5x10-11 and 10-6 m/s, see Contaminant data Fig.1: Extraction of samples from a borehole core for the lab analysis of PCE and TCE. Different sampling and monitoring techniques to determine the depth-discrete contaminant distribution in boreholes have been developed. Depth-discrete sampling is important, since due to a very heterogeneous nature of the aquifer, the concentration can vary strongly over depth. For the planning of a site remediation it is important to know the vertical extent and the location of the contamination, so the remediation system can be planned most effectively. One way to obtain depth-discrete concentrations is to analyze small samples from borehole cores for the sorbed contaminant concentration. An example is shown in Figure 1. However, limestone has a very varying hardness and may be unstable. Soft limestone material is often lost when taking a borehole core. As a consequence, the core analysis is difficult and may lead to wrong results. The following list gives an overview of some other useful sampling methods in wells in limestone aquifers: Snap samplers Diffusion cells Bladder pump Separation pumping with a heat pulse probe The following ones require open (unscreened) boreholes: NAPL-FLUTe (detection of DNAPL) FACT-FLUTe (dissolved concentrations) Passive flux meters for fractured aquifers etc. The following report gives a comparison of some of these sampling methods. Comparison of depth-discrete sampling methods and contaminant measurements at Akacievej (PDF) Return to Content