Check the units before you check the numbers

The most commonly misread figure on a well water report is nitrate, because labs report it two different ways:

Convention Standard
Nitrate-N (as nitrogen) 10 mg/L
Nitrate (as nitrate) 45 mg/L

Same limit. Different units. A result of “nitrate 30 mg/L” is comfortably under the standard if the lab reported as nitrate — and would be a serious exceedance if it were nitrate-N.

People panic over the first and miss the second. Look at the column header before you look at the value.

Lead has the same trap in milder form: 0.015 ppm and 15 ppb are the same number.

The minimum panel

Parameter How often Why
Total coliform bacteria Every year Indicates a contamination pathway into the well
Nitrate At least once Enforceable health limit; infant risk
Arsenic At least once No taste, smell or colour — the only way to know
pH At least once Below 6.5 it corrodes plumbing and rules out some treatment
Hardness At least once Decides whether you soften, and sizes the unit
Iron and manganese At least once Decides the treatment sequence and can foul a softener

Those six answer every question the sizing method asks.

Annual coliform testing is Penn State Extension’s standing recommendation for all private supplies. Minnesota Health’s arsenic guidance is more modest — “at least one time beyond the initial well water test” — because arsenic comes from bedrock and does not fluctuate the way bacteria do.

Two categories of number

This distinction decides whether a result is urgent or cosmetic, and it is routinely blurred by anyone selling filtration.

Health limits — enforceable for public water systems

Parameter Limit
Nitrate 10 mg/L as nitrate-N (45 as nitrate)
Arsenic 0.010 mg/L
Total coliform Fewer than 1 per 100 mL
Lead 0.015 ppm / 15 ppb
Radon 300 pCi/L

Aesthetic guidelines — EPA does not enforce these

Parameter Guideline What exceeding it means
Iron 0.3 mg/L Rusty colour, staining, metallic taste
Manganese 0.05 mg/L Black staining, bitter taste
pH 6.5–8.5 Below: corrosion. Above: deposits, slippery feel
Hardness above 7 gpg / 120 mg/L Scale, soap use, appliance life

EPA states plainly that it “does not enforce” Secondary Maximum Contaminant Levels — they exist “for aesthetic considerations.”

So an iron result several times the 0.3 mg/L guideline means stained laundry and nothing worse. A nitrate-N result barely over its 10 mg/L limit is a different kind of problem entirely. The multiple is misleading; the category is what matters.

What the report will not tell you

Whether your sample was representative. Lead and copper results depend heavily on how long water sat in the pipes — first-draw water in the morning carries the most, and a mid-day sample after normal use can miss a corrosion problem entirely.

Which chemical form a contaminant is in. Arsenic and iron both occur in more than one form, and the form decides which treatment works. A total number does not tell you the species.

Anything you did not ask for. A lab tests what you ordered. A clean report on a six-parameter panel says nothing about the parameters that were not on it.

Once you have the numbers, the sizing method works through the treatment sequence and the arithmetic, and the well water cluster covers each contaminant in depth.