I did a double take also, the unit conversion is a bit confusing... 1 ppb is 1 nanogram per gram but there are 1000 grams in a litre so 1 ppb is actually 1 micr

I did a double take also, the unit conversion is a bit confusing... 1 ppb is 1 nanogram per gram but there are 1000 grams in a litre so 1 ppb is actually 1 microgram per litre. So I still think that approx 20mg/L is the correct figure.