How these water reports are calculated

Every source, formula and threshold we use. Nothing here is proprietary — you should be able to reproduce any number on one of our reports yourself.

1. Where the data comes from

Water chemistry

Texas Water Development Board — Groundwater Database (GWDB). A public dataset of water-quality samples from wells across Texas, published as bulk files and updated nightly. We use the measured parameters listed in section 3.

The wells themselves

Texas Water Development Board — Submitted Driller’s Reports (SDR). Texas licensed drillers must file a report for every well they complete, within 60 days. Depth, tested yield, completion date and location come from that filed report. Each of our reports links to the specific filing for that well.

The limits we compare against

US EPA National Primary Drinking Water Regulations (health-based, enforceable) and Secondary Drinking Water Standards (taste, odour, staining). We set no thresholds of our own.

2. How an area reading is produced

For each parameter we take the median of all published measurements in an area. Median rather than mean, so one contaminated or mis-keyed well cannot move the figure.

reading = median(all published measurements in the area)

area, in order of preference:
  1. a grid square of 0.1° latitude/longitude
     (about 6.9 miles north-south by 5.9 east-west at Texas latitudes)
  2. the county

minimum: readings from 3 DISTINCT wells
  — not 3 measurements. The state re-samples some wells over decades, so three
     readings can come from one well, which is not independent evidence about an area.
     Below the minimum we publish nothing for that location rather than guess.

Hardness is published by the state in milligrams per litre as calcium carbonate. Water-treatment equipment is sized in grains per gallon, so we convert:

grains per gallon = milligrams per litre ÷ 17.1
                  (1 grain per gallon = 17.1 mg/L as CaCO₃)

Each report states how many measurements its figures are based on, how many separate wells those came from, and the years the samples were collected.

3. When we say a reading is high

A report recommends equipment when an area reading crosses a published limit:

The state reports nitrate as NO₃. The EPA limit is written as 10 mg/L measured as nitrogen, which is 44.3 mg/L expressed as NO₃ — that is the figure we compare against, so the units match the data.

Hardness is the one line here that is not an EPA number. The EPA sets no drinking-water limit for hardness — it is not a health or nuisance contaminant in their regulations. The 7 grains/gallon mark is the Water Quality Association’s “hard” classification, which is the industry convention for when a softener is the normal recommendation. Every other row is an EPA limit, linked in section 1.

Health-tier findings are always reported as “area readings exceed the limit — test this well”, never as a statement about a particular household’s drinking water. We are not in a position to make that claim, and we do not.

4. Softener sizing

Sizing figures are straightforward arithmetic from the hardness reading and a stated household assumption. Change the household size and every number scales directly.

water used per day   = people × 75 gallons
hardness per day     = water used per day × hardness (grains/gal)
softener size        = smallest standard unit ≥ hardness per day × 7 days
salt per year        = hardness per day × 365 ÷ 3,500 grains per lb of salt
minerals per year    = hardness per day × 365 × 64.8 mg ÷ 453,592 mg/lb
sodium added         = hardness (grains/gal) × 7.85 mg/L

Assumptions, stated plainly

Worked example

hardness 20 grains/gal, 3 people
  water per day    = 3 × 75 = 225 gallons
  hardness per day = 225 × 20 = 4,500 grains
  softener size    = 4,500 × 7 = 31,500 → 32,000 grain unit
  regeneration     = 32,000 ÷ 4,500 = about every 7 days
  salt per year    = 4,500 × 365 ÷ 3,500 = 469 lb

5. What these reports are not

• Not a test of any specific well. Groundwater varies property to property, sometimes sharply. Our figures describe the area, not a household’s tap.
• Not a safety determination. Nothing we publish states whether water is safe to drink. Only a laboratory test of a sample from the property can.
• Not current. Published samples span decades; each report gives the year range behind its figures. Water chemistry is generally stable, but it can change.
• Not complete. Where too few nearby wells have been tested, we publish nothing for that location rather than extrapolate.

6. Reproducing this yourself

1. Download the bulk files from the TWDB Groundwater Database.
2. Filter the water-quality table to the parameter and county you care about.
3. Take the median of the values.
4. Compare against the EPA limits in section 3.

If you get a different answer to one of our reports, we would like to know.