Case Study - Multilevel well system improved understanding of groundwater conditions in deep aquifers

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Westbay System delivered detailed data in less time and at a reduced cost

CHALLENGE

Characterizing the plume in a shallow  aquifer is simple, but investigating deeper  aquifers poses several challenges.

SOLUTION

MACTEC evaluated various monitoring  technologies and determined that  Westbay System modular instrumentation  was the only system not limited by  insufficient sample capacity in a single  borehole.

RESULTS

Use of the Westbay System for multilevel  monitoring of the groundwater made  it possible for MACTEC to investigate  the deeper aquifers in greater detail  over much less time and at considerably  less cost than singlescreen wells could  accomplish.

Standard wells proved incapable of accurately monitoring deep aquifers

MACTEC E&C was contracted by the U.S. Army Corps of Engineers to conduct a groundwater  investigation and remediation design at Fort Ord, a former army base near Monterey Bay,  in central California, USA. Carbon tetrachloride had migrated from shallow aquifer to two  underlying aquifers.

Characterizing the plume in the shallow aquifer was easy, but investigating the deeper aquifers  posed several challenges. Located 350 to 500 ft [100 to 150 m] below grade, the deeper  aquifers consist primarily of coarse sands and gravels. Historical flow directions in the deepest  aquifer have varied significantly because of the effects of municipal and agricultural pumping,  which have also resulted in regional seawater intrusion. Even after dozens of standard  monitoring wells had been installed,  there were significant gaps in the data available.

Westbay System was selected for its comprehensive monitoring capabilities

MACTEC evaluated various monitoring technologies and determined that Westbay System*  modular instrumentation was the only system not limited by insufficient sample capacity in  a single borehole. Westbay System wells were selected to provide the most cost-effective  solution to monitoring groundwater at the depths required in addition to other beneficial  features such as quality control that were not available with the alternative technologies.

The Westbay Systems were installed in two phases. In the first phase, six wells were installed  with packers inside cased wells completed with 4-in. [100-mm] PVC casing and multiple well  screens. In the second phase, nine Westbay Systems were installed by direct backfilling in  holes drilled with a sonic drill rig.

Westbay System delivered accurate characterization of  deep aquifer system

The value of monitoring at multiple depths in the same aquifer is  demonstrated by the cross sections overleaf. The upper figure shows  early results of sampling for the distribution of carbon tetrachloride.  The detailed interpretation of the distribution would not be possible if  only one screened interval were positioned at each drilling location.  In particular, if the screens were fully penetrating, the carbon  tetrachloride distribution would be grossly misrepresented. Although characterization of the carbon tetrachloride distribution was  the main objective of the project, the availability of multilevel data also  provided new insight into seawater intrusion in the area. The lower  figure shows the distribution of chloride concentrations, with the west  (left) end of the cross section approximately 1.5 miles [2.4 km] inland  from the Pacific coast of California.

The depth-specific nature of Westbay System monitoring has  significantly advanced understanding of the extent of seawater  intrusion within the site boundary. Although technically not a chemical  of concern for the Army, the chloride concentrations clearly indicate  that density variations must be accounted for in the groundwater  model and proposed remedies.

Because the Army and Monterey County share data, these depth- specific chloride concentrations have greatly increased the county’s  ability to monitor seawater intrusion. Historically Monterey County  relied on data from agricultural wells, for which    h the typical installation  with multiple screens over significant depth intervals can have the  effect of diluting the measured chloride concentration. Whereas  previous data indicate possible chloride concentrations on the order  of 500 mg/L at the site, depth-specific samples from the Westbay  wells contain more than 6,000 mg/L of chloride at the bottom of one  of the main drinking water aquifers in the area. Data from shallower  ports in the Westbay System wells illustrate a textbook version of a  seawater intrusion wedge. Continued monitoring will indicate whether  intrusion is continuing to migrate inland or if the county’s efforts to  mitigate intrusion are effective. 

Use of the Westbay System for multilevel monitoring of the  groundwater made it possible for MACTEC to investigate the deeper  aquifers in greater detail over much less time and at considerably less  cost than single-screen wells could accomplish.

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