NTMWD conducts thousands of tests each month in a state-certified laboratory to ensure drinking water safety, and those results are reported to the Texas Commission on Environmental Quality (TCEQ).

Additionally, TCEQ conducts routine water sampling in the NTMWD and city systems through an independent laboratory to confirm water quality compliance with state and federal standards. The cities we serve also collect their own water samples, which are analyzed in NTMWD’s state-certified laboratory and reported to TCEQ.

NTMWD’s water meets all safety standards required by the TCEQ, EPA, and the Safe Water Drinking Act. Test results are available for the public to review in our monthly and annual water quality reports. The cities and utilities we serve also produce their own water quality reports. See list of resources below – jump to test results.


Only chlorine-based disinfectants are approved by the EPA for secondary water disinfection. These included chloramine (ammonia + chlorine) and free chlorine. EPA requires water treatment facilities to maintain a minimum chlorine level of 0.5 milligrams per liter (mg/l)—or 0.5 parts per million (ppm)—during normal operations and a maximum running average of 4 mg/l (or 4 ppm).

A noticeable presence of chlorine does not mean the water is unsafe. Some people can sense (through taste or smell) chlorine concentrations as low as 1 ppm. Test results in 2018 confirmed NTMWD’s system chlorine levels during the disinfection change period were consistent with the rest of the year and within the annual average required by TCEQ.

TCEQ responded to citizen inquiries in 2018 and published an analysis regarding the quality of water from NTMWD’s Wylie Water Treatment Plant in May 2018. The report concluded:

“Exposure to chlorine, monochloramine and ammonia concentrations measured in the water quality samples collected by TCEQ would not be expected to cause short-or long-term adverse health effects.”


Test Results

During the 2020 temporary change in disinfectant, NTMWD will post chlorine levels from samples taken at the three points where water leaves the Wylie Water Treatment facilities. Cities and local providers monitor levels within their local distribution systems. Chlorine levels in local systems should be lower than the levels leaving the treatment plant.

Test Results: Chlorine Levels at City Water Storage Sites 2022-2023

Test Results: Chlorine Levels at City Water Storage Sites 2021-2022

Test Results: Chlorine Levels at City Water Storage Sites 2020-2021

Test Results: Chlorine Levels at City Water Storage Sites 2019-2020

Test Results: Chlorine Levels at City Water Storage Sites 2018-2019

Test Results: Chlorine Levels at City Water Storage Sites 2017-2018

Test results comparing 2019 chlorine levels at City water storage sites to 2018 results to be posted weekly during the period of March 4 – April 1.

Review average daily total chlorine (mg/L) leaving the Wylie Water Treatment Plants:

Average Daily Total Chlorine (mg/L) Leaving Wylie Treatment Plants – 2023 – real-time data. To see previous data, adjust the slider in the upper right corner.

Average Daily Total Chlorine (mg/L) Leaving Wylie Treatment Plants – 2021

Average Daily Total Chlorine (mg/L) Leaving Wylie Treatment Plants – 2020

Average Daily Total Chlorine (mg/L) Leaving Wylie Treatment Plants – 2019


Disinfection By-Products

Disinfection By-Products (DBPs) form when disinfectants, like chlorine, react with naturally occurring compounds in the water. All commonly used disinfectants form DBPs. The EPA regulates DBPs (Trihalomethanes or THMs and Haloacetic acids or HAA5) at levels that current research considers safe. NTMWD’s use of ozone and chloramine for disinfection actually reduces the DBPs in water that are regulated by the EPA and TCEQ.

NTMWD and TCEQ test results confirm that DBP levels are well within the acceptable ranges EPA considers safe. NTMWD has voluntarily increased the frequency of DBP testing to monthly.

Review monthly independent lab testing results for Disinfection By Products (DBPs) on NTMWD water.

Beginning with July 2023, we will include these test results in our monthly water quality reports. These reports are typically uploaded to the website by mid-month for the previous month.

“There may be an increase in the level of disinfection by-products being formed during this short time. Health concerns related to disinfection by-product formation are based on prolonged exposure, and the conversions typically only last two to four weeks at a time. Limited scientific studies following shorter-term exposure to disinfection byproducts have been published that did not find any association between exposure and dermatitis (skin rashes).”TCEQ Facts about Drinking Water Disinfection and the Free Chlorine Conversion Process

“Disinfection byproducts at the concentrations in drinking water would not be expected to cause adverse health effects. The EPA regulates disinfection byproducts such as Trihalomethanes to concentrations sufficiently low so they will not cause health effects.” – Scott Phillips, MD, Medical Toxicologist.

Interview with Medical Toxicologist on DBPs
Medical Fact Sheet
Independent Lab Report on NTMWD System Additional Tests from March 16, 2018


Recommendations if testing as a homeowner