Have you ever wondered how water flows under the ground beneath your feet? We build a low-cost groundwater detection system in under twenty-four hours using common materials and tools. Here's what we did, and how you can do it, too.
Top image: Puff the Science Dragon posing with the completed groundwater detection demonstration setup. Credit: Mika McKinnon
Considering the hack day was in California, the ongoing drought was unavoidable inspiration:
- The rules on water-use are changing, with limiting sale of irrigation water to residents sending people to drill wells to rely on groundwater instead. A low-cost groundwater detection system could be used to explore subsurface water distribution, helping people reduce the risk of drilling a dry well or experiencing cost-overruns when the well needs to be deeper than anticipated to reach the water table.
- Monitoring groundwater levels is tricker than monitoring reservoir levels, yet is essential to coordinating how much water can be safely withdrawn without endangering the subsurface. While it would be lovely for this type of data to be provided real-time by supervisory government agencies, that currently isn't possible. A simple, low-cost groundwater detection system could open up groundwater monitoring to citizen-scientists, taking important measurements while raising awareness about what's going on with this buried water supply otherwise hidden from view.
Alice Pevyhouse, Anita Hart, and Jeremy Wong were intrigued by the pitch of building a groundwater monitoring system out of readily-available materials, so with 24 hours on the clock, we started building.
The Science of Groundwater Detection
Geophysics is the science of characterizing the subsurface without actually needing to go underground to look at it. The field is based on detecting physical properties, then using those characteristics to infer the underlaying geological properties. Between the use of inversion mathematics and the wide range of physical properties for geological materials, it can seem a bit like voodoo even to other geoscientists. The most common way of using geophysics to detect groundwater is through measuring the resistivity of the ground: dry dirt will resist the flow of electricity, while saturated sediments will conduct electricity more readily.
When detecting groundwater as a field geophysicist, we use expensive resistivity equipment that is impractical for mass use. But the physics behind how the equipment works is relatively straightforward:
- Plant evenly-spaced electrically-conductive electrodes in the ground. (Other configurations are possible, but we stuck with the commonly-used Wenner array.)
- Inject current into the outer pair of electrodes.
- Measure voltage across the inner pair of electrodes.
- Pair Ohm's Law with geometric scaling to determine the apparent resistivity at a depth proportional to the electrode spacing.
Inject current into outer electrodes A and B while measuring voltage across inner electrodes M and N. Image credit: Rhett Herman
Once you've calculated the apparent resistivity, all that is left is the interpretation: saturated sediments should have a lower resistivity than dry versions of the same geological material. Change the spacing between electrodes to look shallower or deeper to do a vertical resistivity sounding, or move the entire array to take measurements at the same depth in different locations for a lateral traverse.
You can learn more about the physical principles behind groundwater detection in this excellent paper on building a resistivity system to use in university geophysics laboratories and fieldwork.
Building a Low-Cost Groundwater Detection System
The required materials for a resistivity setup are simple: electrically conductive wire and electrodes, a power supply, and a voltmeter.
Required materials to build a resistivity system: wire, electrodes, power source, and voltmeter. Image credit: Anita Hart
Wire is wire. While field surveys use thick, heavy-gauge wire to increase conductivity and reduce noise, for our simple tabletop prototype, we could use electronics wires scavenged from the many other hardware projects going on at Science Hack Day.
All we needed for our electrodes were four identical electrically conductive rods. While in the field, these rods are usually a quarter-inch to a half-inch diameter steel rods, for our test rig we commandeered four uncoated steel flagging pins.
For a small tabletop setup, a normal 9V battery provided enough power to run a detectable current. In larger-scale applications where a more hefty power source is necessary, even a car battery can have enough juice.
While we could use a store-bought multimeter to measure voltage, building our own voltmeter out of an Arduino allowed us to create a system with the capacity to eventually add on data-logging, while also challenging us to learn more about using the microprocessors. We used these Arduino digital voltmeter instructions as a guide, spending an obscenely long time fiddling with wires, debugging deceptively minor modifications, and otherwise making our lives complicated.
Building an Arduino voltmeter. Image credit: Anita Hart
While the rest of the team worked on building our voltmeter, I set out to build us a tabletop test box. I found a plastic box that wouldn't conduct electricity (minimizing weird currents) and would contain water without making an epic mess. That the box was transparent was an added bonus, giving us a cross-sectional view to peek at what was going on "underground."
The box needed to be filled with a homogenous geological material. As we were building our prototype in the urban heart of San Francisco, this proved more challenging than anticipated. After being rebuffed by a nearby excavation site and unwilling to raid a decorative planter, all seemed lost until realizing that yet another neighbouring construction site had a goodly supply of sandbags. The site supervisor was more than willing to contribute to the advancement of science, sending us off with a sandbag to transform our test box into a sandbox.
Measuring Resistivity To Map Groundwater Distribution
With the rest of our materials acquired and equipment built, it was time to test our prototype. We set the electrodes at three different separation distances, collecting data in our totally dry sandbox as a control reference.
Adding a water table to the sandbox to test out our homegrown resistivity system. Image credit: Anita Hart
Next, we carefully poured tap water in a corner of the box to permeate through the sand to create a saturated watertable. Once again, we took measurements at each of the three distances.
The heavy lifting on the math required to translate measurements into apparent resistivity has already been done by others, making it a simple matter of plugging the measurements into pre-established formula.
- Measurement Depth (z): For a Wenner array where the electrodes are evenly spaced at a distance (a), the depth of the measurement (z) is at approximately half the separation distance: z = a/2.
- Apparent Resistivity (ρ): By Ohm's Law, the resistivity across the inner electrodes (R) is measurement voltage (V) divided by the the applied current (I): R = V/I. The geometric scaling factor to get the apparent resistivity (ρ) is proportional to the separation distance (a): ρ = 2πaR.
Happily, we saw the apparent resistivity drop between our dry control sandbox measurements, and once the same sandbox was saturated with water. The prototype was functional!
The low-cost groundwater detection project won the Science Hack Day award for best use of low-cost hardware.
Puff the Science Dragon posing with his Low Cost Hardware award. Image credit: Mika McKinnon
More importantly, it was a proof of concept that we can build groundwater monitoring systems without spending a lot of money or having a high level of technical skill. This same system will be scaled up with heftier wires, electrodes, and power supplies for field testing next year. If it continues to function well, the Arduino could theoretically be modified to also act as a data logger, even further simplifying the process of conducting a geophysical survey to monitor groundwater levels.
This is an experiment you can absolutely try at home. If you don't want to tackle building your own voltmeter, you can use a normal store-bought multimeter. Be careful when mixing electricity and water to not zap yourself! If you try it out, make sure to tell us about your experiments!