Geographic Variability of Likely Salt Precipitates from Groundwater Wells across the United States

Background

Desalination of hard and brackish groundwater can provide potable water to areas where fresh water is scarce.  However, the desalination process can only recover 50-75% of the treated source water.  The remaining 25-50% is highly concentrated with salts removed from the desalinated water.  This leftover “concentrate” can be discharged to the ocean in coastal areas, but inland areas must find other solutions because salinity levels exceed most the allowable values specified by NPEDS permits. 

One novel solution to the concentrate disposal problem is salt separation and reuse.  Salts removed from concentrate may be valuable to local industry, and best case scenarios are those in which profits from salt sales cover the expense of the removal technology.  Additionally, recovered water can be used to augment current supply. 

This project employed database tools to calculate likely salt precipitates from groundwater across the United States and ArcGIS to search for spatial patterns within the data.

Research Questions

  • Which salts are likely to precipitate first in each well?
  • Are there spatial patterns in the way likely salt precipitates vary in relationship to geography, aquifer type, pH, and distance from surface water body?

Process

  •  Download well data USGS National Water Quality Assessment Program (NAWQA)
    • Parameters: alkalinity (mg/L as CaCO3), pH, Ca2+ (mg/L), Cl- (mg/L), Mg2+ (mg/L), PO43-, and SO42-
  • Imported data to MS Access to calculate likely precipate..
  • Imported likely precipitate data into ArcMap
  • Imported possible co-variant data layers into ArcMap
  • Used ArcMap tools to reveal patterns in likely salt precipitates based on region, pH, aquifer type, and distance from a water body.
Results - Click on Images to Enlarge
Likely Salts
  • No discernable pattern in distribution
  • Calcium sulfate is dominant precipitate
  • Calcium carbonate also frequent

Near Water
  • Wells near surface water are unlikely to be chosen for desalination
  • Most wells are more than one mile from any surface water
Aquifer
  • Wells located in carbonate rock aquifers are more likely to precipitate calcium carbonate.
  • Predominant likely precipitate was calcium sulfate in all aquifers
pH
  • Low pH conditions predominate the Eastern region.
pH Graph
  • CaCO3 precipitates mostly in high pH conditions
  • CaSO4 precipitates regardless of pH.
Regions
  • United States is separted into 9 regions. 
Region Graph
  • No difference between regions can be observed from map or graph

 

Conclusion and Future Research

  • First, Ca2+ is a component of the majority of the precipitates likely to form first, regardless of region, aquifer type, or pH. 
  • This cation will likely need to be removed from the system prior to treatment.
  • The next logical step in the analysis would be to determine the entire sequence of precipitation for each well. 
  • Wells very close to a water body can be removed from future analysis.  Differences between the two types of wells may be more apparent when more soluble salts are considered. 
  • Geographic analysis revealed that pH tends to be lower in the Eastern region of the US, so CaSO4 is likely to be the first precipitate in that region
  • Future work will focus on more complete characterization of the chemical systems, regional market analysis, and the development of a model for analysis of smaller areas
Acknowledgements