The Importance of Collecting Representative Water Quality Samples and How to Do it

One of the purposes of monitoring wells is to monitor subsurface water (groundwater) quality in the saturated zone. Obtaining representative water quality samples is vital, as often the monitoring wells are used to characterize a groundwater contamination issue.

IMPORTANCE OF WATER QUALITY COLLECTION DATA

A key means to understand the extent and magnitude of groundwater contamination, and potentially the means to remediate the contamination, is to characterize the aquifer water quality spatially within a monitoring well network. Without a monitoring well network, it is difficult to adequately understand key parameters that are necessary for evaluating the overall water quality in an aquifer system over a desired areal extent, as well as the spatial and temporal variations in water quality that may be caused by:

• Heterogeneities in the aquifer that affect the fate and transport of contaminants;

• Magnitude and direction of groundwater gradients;

• Layering within the aquifer that may provide preferential flow paths;

• Parameters that are not very soluble in water and may be lighter than the density of water (LNAPLs) and parameters that are denser than water (DNAPLs).

These factors need to be understood as part of the process of collecting representative water quality samples, as well as the constituents of concern in a sampling situation, because they can affect how you sample and for what parameters you sample. However, in all cases care must be exercised in sampling monitoring wells so that representative aquifer water quality is obtained by the sampling event.

SAMPLING PROCEDURES

Typically, a periodic monitoring schedule is established for a site or individual well. For example, samples may be collected semi-annually, quarterly, or monthly. To monitor the groundwater, it is essential that a representative sample is collected. A representative sample is one that does not contain any stagnant water or filter material from the well casing so that characteristic aquifer water is collected. The method used to remove suspended materials and stagnant water is known as “purging” the well. Purging involves removing a minimum predetermined amount of water from the well prior to collecting water samples, as well as monitoring specific water quality parameters for stability. Therefore, it is important to develop effective and accurate groundwater sampling procedures and techniques to provide accurate samples for laboratory analysis.

The first step is to put on nitrile gloves and clean the equipment that will be used to collect the sample. Various equipment can be used to collect water quality samples, such as a bailer, or submersible pump. Whichever equipment is used, it is important that this equipment is thoroughly cleaned prior to inserting it into the well and in-between each well. For example, equipment can be cleaned with a product such as Alconox, or deionized (DI) water depending on the protocol for the project. Following this procedure will minimize the possibility of cross-contamination between wells and the collection of non-representative water samples. Additionally, the analytical laboratory that will be doing the water quality (WQ) analyses will provide the sampling bottles, labels, a cooler to transport them in, and a Chain of Custody (COC) form. The COC is the legal document that states who did the sampling, what the water is being tested for, as well as client information. It is good practice to fill out the labels for the bottles, as can be seen in Figure 1, and COC, prior to heading out to sample the wells.

Next, the position of the purging equipment depends on the expected yield from the well. In a low-yield formation the samples are collected from the bottom of the screened interval. In moderate to high-yield formations the purging equipment is set near the top of the screened interval to ensure that fresh formation water will move upward in the screen. At this point it is important to determine how much water needs to be removed prior to sampling. This is usually site-specific but it typically requires pumping or bailing a minimum of three casing volumes prior to sampling. The method used to measure three casing volumes uses the total depth of the well (TD) and the static water level (SWL). The difference in the TD and SWL provides the height of water in the well. This value can be used with a conversion factor to determine how many gallons need to be purged from the well. For example: If a well has 6 feet of water from the TD to the SWL and it is a 2-inch diameter well, 6 ft x 0.16 gal/foot = 0.96 gallons, which is one casing volume, multiply this by three and the total equals 2.88 gallons. In this case, I would purge a minimum of three gallons, to be safe. This is a simple example; however, in many cases the water in the annular space outside the casing and screen also has to be considered in estimating the minimum purge volume. Some low-yield wells may not be able to produce the required amount of water by continuous pumping or bailing. In this case, the well is purged to dryness and then allowed to recover. The number of times this needs to be done depends on the volume that is extracted each pumping/bailing cycle and the stabilization of the field water quality parameters. After the cleaned equipment is inserted into the well and positioned at the desired height, purging may begin to remove the minimum volume.

Figure 1: Bags filled with water quality (WQ) bottles labeled and ready to go the day before sampling.

Figure 2: The YSI Pro + water quality meter probe collecting data during the purge.

Figure 3: A YSI Pro + WQ meter readout used to monitor various parameters for water quality stability.

The purge volume isn’t the only parameter that must be monitored while sampling. The pH, specific conductance, and temperature must be measured and recorded during the purge to ensure the water is representative of the formation water. A photo of the WQ probe used during the purge is shown in Figure 2. Therefore, make sure you have a water quality meter like the one in Figure 3 that provides a digital readout for each of the field water quality parameters. At least four consistent readings of all three parameters with less than 10 percent variation between readings will indicate stabilization of these parameters.

Once the required amount of water has been purged and the three parameters have stabilized, the water samples may be collected using nitrile gloves. In general, bottles should be filled to one inch below the top of the container to allow room for expansion, and also to prevent the overflow and loss of any preservatives in the bottle. When sampling for organics, pop-top bottles are used and require that the container be completely filled with no air. These bottles are similar in shape to a test tube and have a lid that pops up when properly filled and tightened. Inverting the bottles can show if the sample has been collected with no headspace inside the bottle, i.e., no bubbles.

All water quality containers must be labeled with the sample collection time and date and put in the cooler with ice immediately after collection and kept there until delivery to the lab. Upon arrival at the lab, the cooler temperature will be checked and the COC will be completed. The only task remaining is to wait for the results.

Monitoring well water quality sampling is an essential part of determining water quality and characterizing the aquifer around the well. At LWS we are very experienced in all types of water quality monitoring, as well as designing and installing monitoring well networks to characterize a groundwater system. Please feel free to contact us at LWS (303-350-4090) if you have any questions about monitoring well networks and sampling of these wells.

Bruce Lytle, P.E., Principal bruce@lytlewater.com

Chris Fehn, P.E., P.G., Senior Project Engineer chris@lytlewater.com

Dan Rowe, Staff Engineer dan@lytlewater.com