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Gilbert's Water in Compliance with Safe Drinking Water Requirements

Results of the quarterly compliance sampling for TTHMs conducted on Tuesday, October 17th confirms that Gilbert’s water is in compliance with all Safe Drinking Water requirements.

Post Date:11/13/2017 4:41 PM

This summer, Gilbert notified the public that the annual average for total trihalomethanes (TTHMs) in Gilbert’s drinking water exceeded the maximum contaminant level at one of eight locations sampled quarterly to assess TTHM compliance.  This exceedance occurred at sample site 01-A, located near the intersection of Gilbert and Ray Roads inside Gilbert’s “Zone 1” service area. Compliance with the drinking water standard of 80 parts per billion (ppb) for TTHMs is determined based on an average of quarterly samples collected in the past year.

TTHM is a carcinogen and considered by the EPA to have health effects based on long-term consumption. The Safe Drinking Water standard is calculated to protect consumers from the potential health effects of drinking two liters of water per day for 70 years. TTHMs have been associated with negative health effects such as cancer when water is consumed for that time period at concentrations above the standard.

Since learning of the violation, Gilbert has implemented several immediate responses to ensure that TTHMs return to target levels. Measures include:

  • Using groundwater wells to blend TTHM-free water in the entire system
  • Flushing at strategic locations within Gilbert’s water system
  • Chemical adjustments at water treatment plants
  • Operational changes at surrounding water reservoirs
  • Additional monitoring for TTHMs at multiple locations in Gilbert
  • Utilizing a consultant engineer to recommend TTHM mitigation strategies at Gilbert’s North Water Treatment Plant
  • Procurement of equipment allowing for in-house testing of TTHMs in Gilbert’s water quality laboratory
  • Ongoing development of a water model to optimize system operations

Collectively, these strategies quickly reduced TTHMs at sample site 01-A and system-wide to levels that would ensure Gilbert’s return to compliance after the October 17th sampling.  On October 17th, the sampling at site 01-A was 45 ppb, resulting in an average of 68 ppb at 01-A for the four most recent quarters. All other testing locations including the other locations in Zone 1 remain below 80 ppb as shown below.



January 2017

April 2017

July 2017

October 2017

Running Annual Average


Lindsay Rd & Ray Rd

64 ppb

79 ppb

82 ppb

45 ppb

68 ppb


Val Vista Rd & Ray Rd

43 ppb

45 ppb

62 ppb

62 ppb

53 ppb


Lindsay Rd & Guadalupe Rd

2.5 ppb

30 ppb

64.5 ppb

56 ppb

38 ppb


McQueen Rd & Guadalupe Rd

79 ppb

77 ppb

43.5 ppb

38 ppb

60 ppb


Gilbert Rd & Guadalupe Rd

58 ppb

55 ppb

63 ppb

51 ppb

62 ppb


Greenfield Rd & Guadalupe Rd

60 ppb

73 ppb

54.5 ppb

55 ppb

61 ppb

Gilbert’s water division continues to work hard to ensure that Gilbert meets its goal of 100 percent compliance with Safe Drinking Water requirements. This is being accomplished through assessment and optimization of existing systems and identification and planning for future needs. Gilbert’s water division collects over 5,000 drinking water samples annually from which its laboratory produces over 50,000 analytical results. Gilbert uses this data to ensure compliance with federal drinking water standards for 96 individual contaminants or groups of contaminants.  Gilbert monitors samples at two drinking water treatment plants, 17 active drinking water wells, and 150 locations within its drinking water distribution area. For more information on Gilbert’s water quality, read the latest Water Quality Report.

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