Determining the existence and thickness of floating or sinking hydrocarbons in contaminated aquifers is a very important consideration during a remediation project. Product thickness assessment is critical in the estimation of volumetric calculations and in subsequent selection of recovery method, and expected time frame for the recovery of free product.

A number of products and methodologies are available for free product detection and thickness determination. Among these are: hydrocarbon indicator strips or paste, clear bailers, float/conductivity meters, density sensors, light absorption meters and opto-electric indicators.

Each method has its positive and negative features. This article outlines the operation benefits and limitations of each.

A field technician prepares to monitor product level.

Indicator strips and pastes are materials which are permeable to organic solvents and oil, but impervious to water. When they absorb a petroleum product, they indicate this by a change in colour. Typically, these materials are used by lowering them a known distance into a monitoring well. It is left in place for several seconds, then retrieved. At the surface, the distance is measured from the datum point at surface to the starting and end points of the colour change on the indicator strip or paste. The difference in these two measurements indicates the product thickness.

This method is inexpensive, but the paste introduces an additional chemical to the groundwater. It cannot detect sinking product in wells which also have floating product. Repeat measurements are difficult because either a new tape with indicator paste or a new indicator strip must be used each time, due to the time delay involved in cleaning the paste off the tape, or removing the hydrocarbon which has absorbed onto the indicator strip.

The use of clear bailers to determine product thickness involves the collection of a sample through the floating product and into the water. A clear bailer is slowly lowered into the borehole to a predetermined level, such that the base of the bailer extends a minimum of 4" into the water. The bailer must be long enough to cover the entire thickness of the floating product. When the bailer is retrieved, the floating product is visible and the thickness can be measured.

The main limitations to this method are the difficulty in precise placement of the bailer to intersect the product/water interface, and the decrease in accuracy due to the tendency of viscous product to bypass the bailer entry point, thus resulting in less product in the bailer than actually exists in the monitoring well.

The liquid surface in a monitoring well, whether water or product, is located with a float device which indicates when the probe has been inserted through a liquid. Water is differentiated from product through a conductivity circuit which operates at the same time and level as the float device. If the liquid is conductive it is assumed to be water, and if it is non-conductive, it is determined to be product.

This type of device has accuracy limitations due to its dependence on the density of the liquid it is inserted into. Floating product of a high density will indicate a greater thickness than an equivalent thickness of a lower density product.

Since there is a measurable difference in Light Non-Aqueous Phase Liquid, (LNAPL), e.g. gasoline, water and Dense Non-Aqueous Phase Liquid, (DNAPL), e.g. dry cleaning fluid, densities, a density meter can locate the interfaces between these immiscible liquids. A very useful feature of this technology is its ability to detect the existence and thicknesses of different types of products of different densities in the same well.

The draw back to this type of device is the high expense and the requirement for analysis of output data to identify water level and water/product interfaces.

This technique involves the measurement of the percentage of water in a liquid based on light absorption properties. Product indication results from a zero percent water content, and water is indicated by a 100% reading. A useful feature of this type of device is its ability to detect emulsions.

There can be some questions regarding the actual interface level between product and water when there is a percentage of water mixed with the product. Generally, this type of device is well suited for use in sludge level indication.

By far the most tested and widely used method of oil/water interface detection and measurement is a combination of optical (refraction) and electrical conductivity measurement circuitry.

Typically, an infra-red emitter and detector is used to determine liquid level in a monitoring well. As soon as the probe reaches a liquid, the infra-red beam is refracted away from the detector causing the circuit to indicate a liquid level. A conductivity circuit built into the probe at the same level is used to differentiate between water (conductive) and product (non-conductive). The conductivity circuit and infra-red circuit combination allows detection of air-liquid, floating product-water, and water-sinking product interfaces. The thickness of each liquid is obtained by subtracting the differences between subsequent interfaces as measured on the calibrated lowering tape.

The comparative features of the various hydrocarbon product measurement devices described in this article, pertaining to field use, are shown in Table A.

The selection of a product/water interface device is based on the requirements of the monitoring project. The variety of devices described in this article indicates the need for technologies which have specific functions to suit different uses.