Obtaining Representative Ground Water Samples in a Fractured and Karstic Formation

Ground water was sampled from a number of wells along recharge pathways between fractured shale and karstic carbonate formations to determine the effect of sampling methods on the colloid abundance and the chemical and hydrologic mechanisms controlling the abundance of ground water colloids. Low‐flow minimal drawdown purging and sampling produced samples with much lower and more consistent turbidity than had been obtained during compliance monitoring of the same wells using much higher flow rates and purging of three well volumes. Although some low‐yielding wells required very low pumping rates (10 ml/min) to prevent drawdown, water quality parameters (temperature, pH, electrode potential, dissolved oxygen, specific conductance, and turbidity) stabilized fairly quickly. The total volume of water removed during purging was small (usually only a few liters), and did not appear to be related to either the casing volume or the screen volume of the well. This lack of correlation may indicate that a limited volume of the formation is sampled at the low flow rates. Since only a portion of long screened intervals appear to be sampled, care must be given to placement of the sample tubing to access zones where contaminant migration is expected.