Groundwater Flow and Transport over Larger Volumes of Rock: Cross-Hole Hydraulic and Tracer Testing USEPA-USGS Fractured Rock Workshop EPA Region 2 14 January 2014 Claire Tiedeman and Allen Shapiro, USGS

 Identify flow and transport pathways and barriers between boreholes.  Use of this info with geologic framework helps identify locations of connected high-K fractures and lower-K rocks.  This characterization data is important to developing the site conceptual model.  Quantitative analysis of test data yields estimates of K values. Purpose of Cross-Hole Hydraulic Testing (also called aquifer testing, pump testing) Cross-Hole Hydraulic and Tracer Testing2 Fast Response No Response

 In fractured rocks, hydraulic responses can travel long distances in short times.  Drawdown will not necessarily decrease with distance from pumped well. Expectations: Cross-Hole Hydraulic Tests Cross-Hole Hydraulic and Tracer Testing3

 Well spacings and open intervals  Because of high degree of heterogeneity, difficult to predict distances over which permeable fractures are connected, prior to drilling wells.  Bedding-plane-parting fractures and karst features likely well- connected over longer distances than fractures in crystalline rocks.  Use multiple criteria when selecting locations of new wells – value for characterizing contamination as well as for hydraulic testing.  Multilevel wells important for understanding vertical connections.  Testing  Monitor water levels in as many wells as possible.  Pump & treat system provides excellent opportunity for using remediation activities to further characterize hydrogeology. Designing Hydraulic Tests Cross-Hole Hydraulic and Tracer Testing4

 Procedure:  Shut down pump in one well of the P&T system.  Monitor water-level rises in obs wells.  Start with relatively short tests (run test during the day, with overnight recovery)  Repeat for all wells of system  Advantages  No additional contaminated water withdrawn  Relatively short tests limit effect of shutdown on offsite contaminant migration Using Existing Pump & Treat System for Cross-Hole Hydraulic Testing Cross-Hole Hydraulic and Tracer Testing5

Well Shutdown Testing in Sedimentary Rocks: An Example Cross-Hole Hydraulic and Tracer Testing6 1 st Day: Shut down 45BR 2 nd Day: Shut down 56BR 3 rd Day: Shut down 15BR rain

24BR open to bed ‘301’ Well Shutdown Testing in Sedimentary Rocks: An Example Cross-Hole Hydraulic and Tracer Testing7

 In heterogeneous fractured rock aquifers, analytical solutions for estimating K or T from hydraulic tests have limited applicability.  Need to use numerical model (e.g. MODFLOW) so that heterogeneity can be properly represented. Analyzing Cross-Hole Hydraulic Test Data Cross-Hole Hydraulic and Tracer Testing8

 Process of developing and calibrating gw flow model helps advance the 3D hydrogeologic conceptual model.  Enables consistent synthesis of site characterization data – geology, geophysics, hydraulics.  Provides a tool for understanding the 3D effects of changes to the flow system, such as increased pumping.  Can be updated as new data collected  Starting point for transport modeling Value of Modeling for Interpreting Hydraulic Test Data Cross-Hole Hydraulic and Tracer Testing9 Hydraulic Conductivity Representation Simulated Drawdown

 Valuable for identifying:  Paths for relatively rapid contaminant transport  Less permeable volumes of rock where slow advection and diffusion likely dominate transport  Interpreting hydraulic test data:  Incorporate heterogeneity! Use geology!  Presence of permeable high-angle fractures might be inferred from data, but can be difficult to identify their locations  Tracer testing provides more definitive characterization of transport processes Cross-Hole Hydraulic Tests in Fractured Rocks: Final Thoughts Cross-Hole Hydraulic and Tracer Testing10

Cross-Hole Hydraulic and Tracer Testing11 Application of Tracers Tests for Decision Making at Sites of Groundwater Contamination  Hydraulic testing monitors fluid pressure responses (hydraulic connections)  Groundwater velocity or processes affecting chemical residence times and chemical dilution are difficult to infer from hydraulic information alone (especially in fractured rock)  If information on residence time and dilution is needed... conduct a test that provides a direct measure of these quantities  Groundwater velocity, v  Estimates of fracture porosity (n) from mechanical considerations are unreliable  Hydraulic conductivity (K) is usually an order of magnitude estimate  What are the errors in estimating the hydraulic gradient (  h) in highly heterogeneous aquifers ?

Cross-Hole Hydraulic and Tracer Testing12 Characterizing Residence Time and Dilution Designing the injection of remediation amendments CVOC Conc. (micrograms per Liter) Continuous pumping Feasibility of bioaugmentation in strata between 36BR and 15BR... What is the travel time and dilution from 36BR to 15BR? Hydraulic communication between monitoring locations is a prerequisite for conducting tracer studies...

Cross-Hole Hydraulic and Tracer Testing feet Bromide concentration - February 2008 (7 months after injection) Tracer injection Pumping Characterizing Residence Time and Dilution Designing the injection of remediation amendments Simulations demonstrate slow advection (low K) is responsible for mass retention at greater depths in dipping beds Naval Air Warfare Center West Trenton, NJ

Cross-Hole Hydraulic and Tracer Testing14 Processes Affecting Chemical Migration Estimating parameters governing matrix diffusion Characteristics of breakthrough curves can be used to identify the significance of chemical processes Simulations for different effective diffusion coefficients Storage of tracer solutions at land surface BRP3 15BR 71BR 70BR 25BR 90 ft Tracer Injection Pumping Bromide Concentration vs. Time at 15BRBromide Recovery at 15BR Naval Air Warfare Center West Trenton, NJ

Cross-Hole Hydraulic and Tracer Testing15 Tracer Test Design  Tracer tests can be conducted over various distances (provided hydraulic perturbations are observed between locations)  Single-hole tracer tests can also be designed (point-dilution, push-pull, etc.)  Tracer tests can be designed over hours, days, months or years  Various types of tracers are available to quantify processes of interest (inert, reactive, varying free-water diffusion coefficients, dissolved gases, bacteria, colloids, microspheres, etc.)  Maintaining the geochemical signature of the ambient groundwater (oxic, anoxic, fluid density)  How much tracer mass must be added to register and interpreted a response ? Preliminary estimates of dilution and attenuation are difficult to derive in fractured rock (In some cases, tracer tests are not run; they are re-run!)  Effect of fluid density in structured media  Designing injection apparatus at land surface, and apparatus in injection and monitoring boreholes– maintaining geochemical conditions of ambient groundwater; volume of fluid in boreholes may be large relative to fracture volume; borehole volume may dilute tracer responses Tracer arrival at monitoring wells (> 5 km from tracer injection) in the Madison Limestone

Final Thoughts Controls on Fate, Transport, and Remediation of VOCs16  Tracer tests can provide a direct measure of fluid velocity, chemical dilution and attenuation processes  Tracer tests provide valuable information to (1) conceptualize processes affecting fate and transport of contaminants, (2) design and implement remediation strategies  Tracer tests in fractured rock are likely to be successful under hydraulically stressed conditions, provided the hydraulic perturbation can be monitored in cross-hole tests  Single-hole tracer tests are used primarily to estimate in situ process and identify local parameter values...less reliable in estimating groundwater velocity