1. A Look Back at Groundwater Geochemistry as an Exploration Tool for Lead and Zinc Deposits in Sinking Valley
82 Field Conference of Pennsylvania Geologists
Pre-Conference Field Trip - October 5, 2017
Joe McNally, P.G.

2. Study Objective
Deeply buried or “blind” deposits are difficult to discover
Groundwater chemistry advantage:
May reflect chemical composition at depth
Provide clues to the location of the ore body
Objective: Investigate application of groundwater geochemistry as an exploration tool for Appalachian-type lead and zinc deposits in Pennsylvania

3. Study Areas Four field areas selected
Zones of expected anomalies identified in each area
Domestic wells or springs identified for sampling
One background area: Franklin County with no known lead or zinc deposits (102 domestic well or spring locations were sampled)
Three mineralized areas: (62 sampling locations)
Sinking Valley (16), Bamford (18) and Gap (28)

7. Field Measurements and Lab Analyses
Well construction, depth, groundwater levels
pH, temperature, specific conductance, dissolved oxygen, redox potential, bicarbonate
Samples were field filtered and acidified
Lab Analyses (17 parameters)
Major cations and anions
Trace metals (Fe, Mn, Sr, Ba, Cu, Pb, Zn)

10. Data Analysis Select indictor parameters
lead (Pb), zinc (Zn), barium (Ba), strontium (Sr). fluoride (F) and sulfate (SO4)
Calculate single-element thresholds using Franklin County data
Calculate saturation indices (SI) for each sample location using WATEQFC

13. A word about saturation indices
SI = log (IAP/Ksp)
SI = saturation index
IAP = ion activity product
Ksp = solubility product
SI <1, undersaturated ; SI >1 saturated
In general, as SI increases, the concentration of the dissolved metal ions also increases. This increased concentration of metal ions may be due to the nearby location of mineralized host rock.

17. Study Conclusions
Best single indicators = barium, strontium and fluoride
SI values do not need to be close to saturation to be useful
Si results useful and similar to single-element parameters for identifying anomalies
Well depth alone is insufficient to identify deep circulating groundwater

18. More Study Conclusions!
Dilution most likely source of down-flow decay from anomalies
Down-flow groundwater sample locations should be within 1 km of each other
Domestic plumbing contributes to elevated zinc and copper concentrations. This can be reduced by purging before sampling

19. Study Recommendations
Evaluate rates of dispersion and decay
Establish geochemical criteria to distinguish deep and shallow groundwaters
Conduct field investigation to evaluate zinc anomalies is Franklin County

20. Some Thoughts Field sampling and data evaluation methods still apply
Both pros and cons for using domestic wells as sample locations
PROS: Easy access and low sampling cost
CONS: Lack of 3-dimensional specificity
Sample collected is likely a composite of several water bearing zones at multiple depths rather than a sample from a discrete interval
Influences from plumbing

21. More Thoughts! Some Questions:
Can new tools/methods (e.g. stable isotopes, age dating, new computer modeling) be applied to obtain information about “deep” vs “shallow” samples?
How sensitive are results to seasonal variation? While absolute values may change, does this affect the relative differences used to identify anomalies?
A lot easier now to prepare maps and figures with GPS and GIS supporting programs! More graphics (i.e., Piper, Stiff, etc.) diagrams could be useful.

22. Contact and Source Information
Contact Information:
Joe McNally, P.G, Principal Hydrogeologist
GeoServices, Ltd.
1525 Cedar Cliff Dr., Camp Hill, PA
(717)
Source
Application of Groundwater Geochemistry as an Exploration Tool for Carbonate-hosted Zinc-Lead Deposits in Pennsylvania. Joseph McNally, Penn State, MS Thesis (1984).