As a consultancy for soil and water we apply a lot of different sampling techniques for organic and inorganic substances, nutrients and toxics in all environmental compartments, e.g. under exclusion of air or using isotopes etc.
A unique feature for the measurement of groundwater recharge quality is available only with us:
The Self-Integrating Accumulator Method (SIA method) measures the total losses of a wide range of chemical substances from the root zone into the groundwater. It does not interfere with superficial activities like ploughing and samples all solutes of interest from the soil leachate.
Apart from this, your question always rules the procedure: Does your tap water comply with legislative or health criteria? How does agricultural management affect groundwater quality in the long run? Time and scale affect grossly the most appropriate sampling strategy.