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Understanding Chemical Analysis Results

How to read and interpret the results of chemical analyses reported to public water systems by a drinking-water laboratory.

Chemical analysis results can, without question, be confusing. However, understanding the basic components can help make sense of it. While there are many types of analysis results there are many components common to all of them.


Many dates appear on the analysis form, but only one is of real interest to the water system. This is the date the sample was collected. On the result form, this date is called the “Sample Date,” “Collection Date,” “Source Date,” or “Date Collected.”


These are characters found before the results number. There are two common qualifiers, “<” (less than) and “>” (greater than). When you see a “<” sign in the result, the amount indicated is the smallest amount that the analysis method can detect. Therefore, when a “<” sign is in your results, no detectable amount of that chemical has been found.

It happens rarely, but you may also see a “>” sign in the result. This means the amount of chemical found is above the amount the analysis method can measure accurately. You have at least the amount indicated and probably more.

Units of Measure

A unit of measure is the amount of chemical found in a specific volume of water—for example, pounds/gallon or teaspoons/gallon. It is also called concentration units, or simply units. The four common units of measure used in drinking water analysis results are mg/L, µg/L, ppm, and ppb.

Milligrams per liter (mg/L) is equivalent to ppm (parts per million). To convert mg/L to µg/L simply multiply by 1000 (1 mg/L = 1000 µg/L). One part per million is about the same as one drop of soda in fifty 32-oz. drinks.

Micrograms per liter (µg/L) is one thousand times less than mg/L and is equivalent to ppb (parts per billion). To convert µg/L to mg/L, simply divide by 1000 (1 µg/L = 0.001 mg/L).

Even though these amounts appear very small, amounts of any size can be very potent and very harmful. For example, trace amounts of pesticides are known to be very harmful at levels 1000 times less than 1 µg/L or 1 ppb, and strong evidence exists that lifetime exposure can result in cancer. Therefore, a very minute amount of chemical constituent can be of significance and concern.

Types of Chemicals Sampled

Inorganic chemicals do not contain carbon. They are metals such as iron or arsenic, minerals such as calcium and fluoride, and salts such as nitrate and nitrite. Many metals, minerals, and salts simply make the water unappealing to drink while others, when found in excessive quantities, have detrimental effects on human health. For example, nitrates in excess of 10 mg/L can cause “blue baby syndrome,” which results from interferences in the blood’s ability to carry oxygen. This can be fatal to infants.(30 TAC 209.106)

Organic chemicals contain carbon and include synthetic organic compounds (SOCs), volatile organic compounds (VOCs), and trihalomethanes (THMs). SOCs are found in insecticides, herbicides, and polychlorinated biphenyls (PCBs). VOCs are found in gasoline, paints, solvents, and plastics, and are generally considered petroleum-related products. (30 TAC 209.107)

THMs and HAAs (haloacetic acids) are chemical by-products that are formed when water is treated with chlorine. They are also referred to as disinfection by-products, or DBPs. Many organic chemicals can cause cancer if consumed at levels greater than the maximum contaminant level (MCL) for many years.(30 TAC 209.113)

Radiochemicals are radioactive materials, which can occur naturally or as a result of oil and gas production or mining. Alpha particles are positively charged particles while beta particles are negatively charged particles. Both types can cause cancer if consumed for many years at levels above the MCL. (30 TAC 209.108)

Microbiological contaminants  include organisms such as coliform bacteria and Escherichia coli (E. coli), and protozoans such as Giardia and Cryptosporidium. Many microbiological contaminants can cause immediate health problems with symptoms similar to flu or intestinal distress. These symptoms are temporary for healthy individuals but can be fatal for infants, the elderly, and individuals whose immune systems are compromised by illness. (30 TAC 209.109)

Sampling Frequency

Sampling frequencies are based on several factors, such as:

  • Type of water system
  • Size of the population served by the system
  • Type of water (groundwater or surface water)
  • Detections and MCL violations
  • How vulnerable the water source is to contaminants


It is the responsibility of the water system to pay for chemical analysis. Failure to do so will result in monitoring violations. The laboratories used by the TCEQ are nonprofit, which means that they only charge for the actual cost of analysis. These laboratories are responsible for billing the water systems and sending you the results. Please direct billing questions to the laboratory. You may contact the Department of State Health Services at 512-458-7318 or the Lower Colorado River Authority at 512-356-6022. If you need assistance with scheduling or interpretation of the results, please contact the TCEQ at 512-239-4691.

Schedule of Laboratory Fees (PDF)

Maximum Contaminant Level (MCL)

The TCEQ reviews all chemical analyses. When an exceedance is found, several things happen:

  • TCEQ will verify the result with a check sample, if necessary
  • The water system will increase sampling for the parameter that exceeded the MCL
  • If a violation exists the TCEQ sends a notice of violation letter to the water system
  • The water system must notify customers of the violation so vulnerable populations are protected
  • The water system must make changes to curtail or correct the violation
  • Summary of MCLs