For trace level analytes, or Ion Chromatography, 18.2 meg ohm water with low TOC is best.
For ppm levels in soils or waste water, a less expensive system may suffice. Distilled or deionized water may be fine for these methods.
Particulate will be a problem for Lachat systems, so water needs to be free of particles. The analyte being tested for should not be present or it can cause upside down peaks (carrier contamination) and/or a low slope.