All hydrogeological processes occur within the framework of a flow system. Consequently, characterizing the flow system is essential to nearly every hydrogeological investigation, from determining contaminant impacts at a water supply well to understanding groundwater-surface water interactions. Although the phrase ‘flow system characterization’ is used frequently it is often not explicitly defined. I define flow system characterization as quantitative descriptions of flow path attributes (e.g., three-dimensional trajectory, associated groundwater residence time, evolution of biogeochemical conditions). Quantification of these attributes is challenging for 3 main reasons, 1) generally, they cannot be directly observed; 2) measurements of hydrogeologic properties and processes are typically sparse compared to the subsurface volume of interest; and 3) the primary data collection instrument, a well or borehole, fundamentally alters the flow system of interest. As a field based, physical hydrogeologist I look for creative ways to collect and interpret field data to overcome these challenges and provide insight into natural flow system conditions.
My approach to groundwater studies involves three broad strategies:
prioritizing data collection from continuous cores, temporarily sealed boreholes, and high resolution, depth discrete multilevel systems to be most representative of flow system conditions away from the altered condition of the borehole
collecting highly resolved spatial and temporal field data to reduce uncertainty
collecting numerous, complementary field data sets to provide multiple lines of evidence to constrain interpretations and conclusions.
I often do my research at contaminated sites where the contaminants serve as tracers of the physical flow system.
Core sampled (pink blocks) for analysis of industrial contaminants in the rock porosity
Waterloo multilevel system - 13 monitoring intervals in one borehole.
Sealing an open bedrock borehole with a FLUTe liner
1. Using hydraulic head profiles as a fundamental diagnostic tool for flow system characterization
Hydraulic head, a fundamental hydrogeologic parameter, is often considered an insensitive parameter within the context of characterizing heterogeneous groundwater systems. This mis-conception stems from the standard approach of collecting water levels from relatively long, often cross-connected, monitoring intervals at only 2 or 3 depths at any given location. These head data are uncertain, biased, and have poor spatial resolution. Highly resolved depth-profiles of hydraulic head are sensitive to changes in hydraulic conductivity (K) often associated with changes in fracture network characteristics. I have applied high resolution head profiles (e.g., 46 monitoring intervals over 130 m of rock) to characterize the variability in hydrogeologic properties with depth in flow systems at three contaminated sedimentary rock field sites (Meyer et al. 2014). The head profiles have a systematic shape indicative of numerous, well connected fractures and their shape is highly repeatable through time. Consequently, I've used these profiles to delineate the position and thickness of contrasts in vertical hydraulic conductivity. Current research projects are focused on:
testing the head profiling technique in a wide variety of geologic/hydrogeologic settings including Silurian dolostones in north-east Iowa and glacial sediments in south central Wisconsin
2. Defining the Relationship Between Hydraulic Conductivity Contrasts and Stratigraphy
3-D delineation of units with contrasting hydraulic conductivity (K), commonly referred to as hydrostratigraphy, is a critical component of many hydrogeological investigations. One common approach is to use relatively sparse field and laboratory hydrogeological data to split or lump lithostratigraphic units or lithofacies into hydrostratigraphic units. This approach takes advantage of the geologic framework to inform the geometry of K units between sparse measurement points. However, there is often little direct evidence the geologic units correspond to separate K units. This lack of corroboration is a particular issue in fractured rock systems where bulk K is controlled by fracture network characteristics rather than the grain size and sorting of the material. An alternative approach is to use high resolution head profiles as direct evidence of K contrasts and then determine what stratigraphic framework best describes the distribution of those contrasts (Meyer et al. 2016). At a site in Wisconsin, a sequence stratigraphic framework was much more predictive of the position and thickens of K contrasts than the lithostratigraphy. Current research projects are focused on:
utilizing flow system tracers to test hydrostratigraphy delineated using head profiles
exploring the relationship between sequence stratigraphy and hydraulic conductivity contrasts in shallow bedrock aquifer systems
3. Advancing Understanding of Shallow Bedrock Aquitards
In porous media, aquitards are conceptualized as intervals of geologic materials with low vertical hydraulic conductivity. However, in rock, bulk hydraulic properties are usually controlled by the fracture porosity, not the matrix porosity. Consequently, poor connection between fracture sets might give rise to aquitard conditions that are distinct from those in granular, porous media. High resolution head profiles collected from the sedimentary rock sites across the US and Canada provide hydraulic evidence for poor vertical connectivity across preferential termination horizons. These head profiles reveal large changes in head occurring across thin (< 1 m) stratigraphic intervals or bed contacts. Comparisons with detailed lithologic information from the cores and geophysical logs suggest these head loss zones are associated with preferential fracture termination horizons but direct evidence is lacking because vertical features are under-represented within vertical boreholes. In cooperation with collaborators and my students, I am collecting data from nearby outcrops to determine if the zones of distinct head loss are correlated with preferential fracture termination horizons. Current research projects are focused on:
evaluating 'surfaces' in unconsolidated sediments and bedrock as important hydrogeologic features
characterizing the response of dense non-aqueous phase liquids (DNAPLs) to preferential fracture termination horizons using high resolution field data
4. Natural Attenuation of Organic Contaminants in Sedimentary Rock Aquifer Systems
A recent NRC (2012) report indicated that there are > 12,000 complex contaminated sites in the US where restoration within the next 50-100 years is not achievable. Fractured rock sites impacted by DNAPL with large plumes are particularly challenging sites. A key component of evaluating potential management and remediation strategies is quantifying the extent of natural attenuation and characterizing its variability through space and time. Much research has been done looking at natural attenuation in sand and gravel aquifer. However, less research has been focused on fractured rock systems. Current research projects are focused on:
quantifying contaminant mass reduction in the rock matrix at a contaminated sediment rock research site and evaluating the relative contribution of attenuation processes to that mass loss
utilizing historic (30+ years) of conventional well data combined with high resolution research data to evaluate natural attenuation of a large (~ 9 km^2) plume in fractured sandstone with the ultimate goal of developing field data based estimates of degradation rates
5. Linking the Broader Flow System to Local Scale Groundwater-Surface Water Interactions
This is a relatively new research theme for our group. Here we are trying to better understand how the larger scale flow system influences hyporheic/transition zone processes in streams and lakes. We currently have two projects within this theme. The first project is focused on improving estimates of water and dissolved phase iron exchange between a glacial aquifer and a ferruginous (iron-rich), meromictic (doesn't turn over) lake in Minnesota. These estimates will be used to improve the iron budget for the lake which is important for long term studies looking at the biogeochemical cycling of iron and carbon in the lake. The second project is focused on characterizing groundwater-surface water exchange and transport of pharmaceuticals and pesticides at several spatial and time scale for a stream whose discharge is dominated by wastewater treatment plant effluent. The results of this study will be used to improve our understanding of differential attenuation of the wastewater derived contaminants in the stream versus in the groundwater system.