What is the difference between low flow groundwater sampling and standard groundwater protocol?
In 1996, the EPA introduced a low flow purging and sampling methodology that is widely used and accepted today. The primary purpose of low flow sampling is to purge and sample wells at rates comparable to ambient groundwater flow so that drawdown of the water level is minimized and mixing of stagnant water within the well casing is reduced. This method also reduces turbidity in the sample which can cause bias resulting in “false positives”. Prior to the release of EPA’s Low Flow Sampling Procedures, groundwater sampling was typically performed by a volume average method using bailers or pumps designed to remove large volumes of water from the well casing as quickly and efficiently as possible. Today, while the fixed volume sampling is still fairly common, it is generally accepted that low flow sampling procedures result in the collection of more accurate and representative groundwater samples as compared to the traditional method.
Volume average sampling or what some might refer to as “standard protocol groundwater sampling” involves evacuating three to five times the volume of standing water within the well casing. Submersible pumps, inertial pumps, peristaltic pumps, or bailers are often used to purge and sample monitoring wells using this method. The logic being that, after removing stagnant water, the well recharges with groundwater as it comes in through the screened interval of the well. During the purging process pH, specific conductivity, temperature, and dissolved oxygen measurements are collected until readings are stable, at which time samples are collected. Using this method, water from the screened interval mixes with water from other parts of the well and samples are often highly turbid. In addition, sampling with bailers and high volume pumps tends to aerate samples, stripping out volatile organic compounds that may be present in the groundwater sample. Another byproduct of this sampling method is the generation of significant quantities of purge water which, in many cases, must be stored in drums prior to waste characterization and disposal.
In contrast to standard groundwater sampling protocol, low flow sampling provides some distinct advantages particularly from a sample quality perspective. This procedure calls for pumping of groundwater at low-flow rates (usually less than 500 ml/minute) from the screened interval of the well. Water levels are measured continuously and if drawdown is observed, the pumping rate is further reduced. The idea is to collect samples truly representative of the groundwater in the screened interval of the well which is representative of water in the aquifer. Because of the low pumping rates, the likelihood of volatile stripping is significantly reduced or eliminated and since sediments in the well are not stirred samples with low turbidity can be collected. Another important feature of the low flow sampling procedure is the measurement of stabilization parameters (pH, conductivity, dissolved oxygen, and oxidation reduction potential) using flow-through cell. This device provides for the collection of more representative measurements because the sample is not exposed to ambient air and groundwater passes more evenly across the sensors. The flow cell more closely mimics the conditions within the well (and aquifer) thus resulting in a more reliable measurement. Low flow protocol outlines specific guidelines as to when stabilization of indicator parameters have been achieved, all of which must be recorded in detailed field logs. Once stabilization of all indicator parameters is observed, samples are collected prior to the flow through cell as to avoid any potential cross-contamination. This groundwater sampling method results in the generation of much smaller quantities of purge water. Depending on specific site conditions, sampling parameters, and well depths, various devices can be used to collect low flow groundwater sampling including: peristaltic pumps, submersible pumps, and bladder pumps.
In general, low flow sampling requires more equipment, personnel training. It is also more time consuming and expensive as compared to volume average sampling. That being the case, for quality purposes, it is the preferred method of sampling among environmental professionals throughout the country.