HSI was hired by the City of Scottsdale to support CDM in the investigation of the cause of “white water” being produced by one of their drinking water production wells.  Prior to testing, the City believed the problem to be caused by cascading water that results from slow but steady drainage of the hydrogeologic units above the production water level when the well is being pumped. The goal of the tests performed by HSI was to validate this cause or determine if the cause was due to gases emitted by the aquifer.  Cascading water in a well will cause air to be entrained as it drops inside the well casing to the rapidly declining water level. Turbulence within the well casing occurs as the water pours from the upper part of the screen to the lowered water level in the well.  The resulting air bubbles formed in the water are predominantly composed of nitrogen and oxygen gases that are characteristic of the air within the well casing. If the aquifer is contributing gases to the water, the bubbles in the water might have a tendency to be greater in carbon dioxide or methane gases.

A more direct method for validating the occurrence of cascading conditions would be to video log the well under pumping conditions. However, this was not possible in COS Well 86 without pulling the existing pump and replacing it with a smaller pump that would allow sufficient clearance for the installation of the video camera. The City chose not to pursue this technique at the time.  Two approaches were used in testing the well for the source of white water production. The first approach was to evaluate a range of field water quality parameters in coordination with monitoring the drawdown in the well. Any trends in the data would then be correlated to well construction information in an effort to prove or disprove cascading water and air entrainment. The field parameters tested included temperature, dissolved oxygen, oxidation-reduction potential (ORP), pH, and conductivity.  The second approach was an innovative method developed between HSI and Del Mar Analytical Laboratories of Phoenix, Arizona that involved the collection of 22 water samples over the duration of the three-hour test to ultimately be analyzed for fixed gases. The samples were collected into 60 mL vials with septa lids having a soft penetratable membrane, and filled so that no headspace remained in the sample bottle upon initial filling. Over a short period of time it was anticipated that tiny gas bubbles entrained within the sample would coalesce and form a large enough bubble to sample with a syringe that could penetrate the septa lid. The fixed gases analyzed from these samples included nitrogen (N2), oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4).  Ultimately, the fixed air analysis indicated a composition similar to air, supporting the occurrence of cascading within the well.

     Water Quality Evaluation

Vidler Water Company contracted the services of HSI to provide water resources consulting services for the design, drilling oversight, and data analysis of an exploration/test well in the Mesquite Groundwater Basin (Sandy Valley) which is located approximately 40 miles southwest of Las Vegas, Nevada. A significant part of the project was devoted to water chemistry analysis, isotope testing, and geochemical modeling to demonstrate that the constructed exploration/test well was withdrawing water from a deeper carbonate aquifer and not the overlying valley fill.  Zonal water quality samples were collected throughout the borehole during drilling.  After drilling of the exploration/test well to 1,502 feet and completing the well in the carbonate rock aquifer, aquifer discharge testing was conducted for 72-hours with water quality sampling.  A monitor well was drilled at the same site to 341 feet, terminating in the alluvial aquifer, where an additional short duration aquifer test was conducted and water sample collected. Several domestic wells were also sampled in Sandy Valley to support the investigation. The results of the water quality evaluation and geochemical modeling were presented with other collected hydrogeolgical information to the Nevada State Engineer at a water rights hearing in support of water right applications submitted by Vidler Water Company. 

SunChase Estrella Limited Partnership hired HSI to evaluate the quality of the groundwater in Rainbow Valley, using historical data, zonal sampling data that HSI collected during the drilling of two exploration holes, and collecting new samples from operating wells on their southern property.  Each well was analyzed for major inorganic constituents, trace elements, and eight of the samples were analyzed for the stable isotopes of oxygen-18 and deuterium.  This first phase of groundwater quality investigation was completed in October of 2001, and resulted in predictions of where better quality groundwater was most likely to occur.  The results of the investigation presented maps of the property that indicated where lower total dissolved solids, nitrate, and fluoride concentrations could be found.  The stable isotope analyses helped to highlight areas more impacted by agricultural irrigation.  From the results of the investigation, HSI was able to identify the best areas for drilling new wells based on water quality, and to provide estimates on quality for the engineering team who was creating the master plan, including water treatment plans.

Additional water quality work was performed for SunChase (now Newland Communities)  between 2003 and 2005 to refine the location of future production wells and recharge wells for the expansion of the Estrella Mountain Ranch development.  This work involved retesting some of the same wells sampled in 2001 and filling in data gaps with samples from other wells that were active on the property.  Comparisons were made to determine if any significant changes had occurred since the previous testing.  Other testing was also performed in association with aquifer tests that were done at three existing wells, strategically located at sites where production wells or recharge wells were being proposed.  Mixing models were then performed to estimate blending of groundwater with RO-treated water to reduce treatment costs, and to estimate the chemical impact to the aquifer of recharging it with reclaimed water.