Archive for June, 2013

Addressing Resolution Limitations of the Dynamic Dye Tracer and Sampling Techniques Used in Production Wells

Tuesday, June 18th, 2013

We are presenting a poster at GRA’s Symposium on High Resolution Tools and Techniques for Optimizing Groundwater Extraction for Water Supply.  Here is our abstract:

Addressing Resolution Limitations of the Dynamic Dye Tracer and Sampling Techniques Used in Production Wells

Cameron Tana1, Nicholas Byler2 (presenting),  David Van Brocklin3, and Derrik Williams2

1 HydroMetrics Water Resources Inc., Potomac, MD

2 HydroMetrics Water Resources Inc., Oakland, CA

3 Perth, Western Australia

The U.S. Geological Survey (USGS) has developed a commonly used technique for characterizing depth dependent flow in water supply wells (Izbicki et al. 1999).  The technique involves injecting a dye tracer at various depths in the well and measuring the time elapsed for the tracer to reach the well head.  Differences in travel times are used to estimate velocity and cumulative flow in the well between two depths.  However, the resolution of the obtained data is limited since the technique only provides average velocities and flows over depth intervals in the wells.  If the data’s resolution limitations are not addressed, important flows or chemical concentrations may be assigned to the wrong depth interval without acknowledging the potential error.

We recommend two field strategies for improving the resolution of the dye tracer and sampling techniques: 1) injecting the dye tracer at different depths within blank casings between screens, as flows should be uniform within the blank and 2) sampling at different depths than the dye injection depths to increase spatial resolution of obtained data.

Data interpretation should address the resolution limits of the techniques by fitting estimated travel times and well concentrations to the raw data.  One approach is to calculate a mass balance that fits the obtained data (Halford et al., 2010).  We will present an example of this approach modified to use data obtained with the two recommended field strategies.  A second approach is to use an axi-symmetric model such as AnalyzeHOLE (Halford 2009) and calibrating to the obtained data.  We will also present an example of calibration for an axi-symmetric model developed based on Langevin (2008).

Halford, K., 2009.  AnalyzeHOLE-An integrated wellbore flow analysis tool, U.S. Geological Survey Techniques and Methods 4-F2.

Halford, K.J., Stamos, C.L., Nishikawa, T., and P. Martin, 2010.  Arsenic management through well modification and simulation, 48, no. 4. 526-537

Izbicki, J.A., Christensen, A.H., and R.T. Hanson, 1999. U.S. Geological Survey combined well-bore flow and depth-dependent water sampler, U.S. Geological Survey Fact Sheet FS 196-99, October.

Langevin, C.D., 2008.  Modeling axisymmetric flow and transport, Ground Water, 46, no. 4, 579-590.

Considerations for Developing Effective Groundwater Recharge Policies

Monday, June 17th, 2013

Derrik Williams gave this talk to Groundwater Association of California’s San Francisco Branch meeting June 17, 2013.

Mapping and quantifying natural groundwater recharge is arguably one of the most important factors in successful groundwater management. Unfortunately, recharge is often very difficult to measure directly. Recharge estimates are therefore often based on assumptions: some which are valid and some which are questionable. These assumptions can lead to, and have led to, land use policies that can be counterproductive to groundwater managers. A clear discussion of recharge mechanisms, physical factors limiting recharge, groundwater quality impacts from recharge, and ecological/habitat impacts from enhancing recharge must precede any land use decisions.

Because groundwater recharge is often viewed in the context of providing urban supplies, we will focus on recharge in urban areas – and land use policies that can enhance this recharge. This talk reviews the current understanding of the influences of urbanization on groundwater recharge from both a quantity and quality perspective. We will compare what is known about recharge with existing policies and recently passed legislation requiring mapping of recharge zones. Based on our understanding of recharge mechanisms, we can outline what hydrogeologists should consider when negotiating with land use planners, and how they should be influencing land use policy.