Eliminating Known Unknowns

One question our hydrogeologists are commonly asked about problem wells is “Where has our water gone?” A very interesting question and one that is much harder to answer then you might think. Our newsletters over the years have discussed some of the ways in which the groundwater industry has changed, but one thing has remained constant-the need for good quality water level data. Robinson Noble has assisted numerous water purveyors with the development and implementation of well field monitoring plans designed to provide good quality data and foster proactive aquifer management. Generally, these monitoring plans have focused on the collection of long term aquifer water levels and production data. However, monitoring plans can also be tailored to a specific well or aquifer concern.
In the context of trying to answer the “Where has our water gone” question, the availability of good quality data plays a crucial role. Many well owners have little to no documentation of the well’s historic production capabilities and, in some cases, no information regarding the well’s final construction. Without an accurate understanding of the well or wellfield’s historical trends, it is difficult to assess the likely cause of a well’s decrease in pumping capabilities or its outright failure. The collection of timely and accurate water level data is essential to proper well diagnosis and forecasting trouble that may be brewing.

So what data should be collected in order to provide the proper tools for a well evaluation? In general, the following information and data should be collected and stored for each well: construction details and the geology encountered during drilling (typically from the Water Well Report filed by the driller or a consultant’s report); post-construction well rating and water quality (consultant’s report); static and pumping water levels and long term production quantities (manual data collection or SCADA systems). If data collection has not been made a priority, it may be hard to come by, leaving purveyors (large and small) scrambling to compile data that has been sporadically collected over the years since the well was drilled. A proactive collection of this data will allow for a faster, less expensive determination of potential well problems.

While there have been many advancements in the ways to collect, store, analyze, and ultimately work with data, the data required has not changed. Without accurate and consistent water level data collection, many of the new tools fail to produce a quality solution. A groundwater flow model, for example, is only as accurate as the data from which it is built. For the purposes of this article we will discuss the two most common methods of water level data collection, manual measurements by system personnel and electronic measurements through the deployment of a data logging pressure transducer. There are a number of pros and cons to each method. For example, manual measurements only require the purchase of well sounder and the time it takes personnel to visit the site; however, the number of datapoints collected is generally limited to the number of times staff visit the site, which may be at an insufficient frequency to diagnose a problem. Data loggers, on the other hand, can collect nearly constant data but may have a high up-front cost (depending on well construction and system specifications). Regardless of the methodology employed, the accuracy of the data is dependent on the level of care administered by the data collector.

When manually measuring water levels, a consistent method should be used. Soundings should be referenced to a consistent datum, like a particular location on the top of a sounding tube or casing, relative to ground surface. If the pump is running, note how long it has been pumping and at what rate; if the pump is off, note the time since pumping ceased. Understanding the state of the well during water level collection provides a reference point for the measurement to be interpreted and compared to other data points. Knowing whether the well has been pumping for two minutes or two hours greatly changes the interpretation of the collected “pumping” water level. Similarly, “non-pumping” water levels collected during a state of recovery may have drastically different depths relative to the time since pumping ceased.

The collection of data using electronic data loggers allows for nearly constant water level data collection. As with manual measurements, the data logger should be referenced to a consistent datum. That datum should be used when collecting any supporting manual measurements at the well. For long-term data collection, data loggers can be deployed to collect a water level measurement at a frequency of 15 minutes. At this sampling frequency, it is less critical to know the exact moment the well began or ceased pumping as it can be estimated to a single 15-minute period. In addition, some data logging equipment can be specifically designed to work with SCADA systems already in place. If a data logger is to be used, sounding tubes should be installed in the well by a licensed drilling contractor to ensure the equipment does not hang up on the power cable or pump column. Typically, sounding tubes with a clear inside diameter of one inch are suitable for this purpose. If room is available in the well casing, purveyors may benefit from installing more than one sounding tube, allowing one to be dedicated to the data logger and leaving the second one free as a backup and for the collection of manual water level measurements.

There is one additional data point which should be collected: a “static” water level. There is often confusion regarding what this term means. A static water level is not simply a non-pumping water level. While it is true that a static water level is collected when the pump is off, it is also necessary for the water level to have fully recovered from any recent production. This can be difficult to achieve in an active well or well field, but is necessary to have an accurate understanding of the aquifer’s long term health and sustainability. If data are being collected manually and a true static water level cannot be measured during routine inspections, a special effort should be made at least quarterly to collect a representative static water level. Depending on the operational design of the water system, it may be necessary to temporarily disable any on-demand pumping systems in order to collect a true static water level. Systems may be able to top off and extend storage quantities, allowing for an extended period of non-pumping and the collection of a true static water level.

The cost of collecting and archiving historic water level data will differ substantially for a residential well and large purveyor that may have multiple wells or well fields. Owners of even a single well can benefit from the collection of water level data and an understanding of how water levels change over time; however, the necessity of having this data is often not evident until a well has a problem.

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