Gravitional Effects on Groundwater Levels

(Part1)

I recently reviewed some high-frequency groundwater level monitoring data from several wells and observation boreholes located within a few kilometres of the Indian Ocean that had been described in a report as demonstrating “tidal” influences. For most people the term “tidal” infers that groundwater levels are being influenced by ocean tides, which raises the possibility of deterioration in water quality by seawater intrusion, if the aquifer is over-abstracted. In this case the aquifer provides the main potable water supply for several hundred thousand people. Consequently, it is worth taking a closer look at the data.

Data Origins

Water levels were measured using In-Situ Rugged Troll Loggers. The data in Figures 1 through 5 are from a logger recording at hourly intervals. The logger was positioned within an observation well located approximately 500 m from several high-volume abstraction wells. The aquifer is primarily a loosely consolidated sandstone sequence over 200 m in thickness. The monitoring well is 90 m deep, 80 m of which are below sea level. It is constructed with a 10 m slotted screen at its base. Water levels are within 11 m of ground level.

 

Figure 1: Groundwater Levels and Barometric Pressure (Jan to Jul 2018)

Rainfall Recharge

Wet season rainfalls commenced in mid-April, with the main monsoon arriving in early June. The resulting recharge is reflected in a corresponding steady and increasing rate of rise in groundwater levels (Figure 1). In the six-month monitoring period, water levels showed a maximum-to-minimum range of less than 2.5 m. The specific yield has been estimated at between 20 and 30 percent.  Maximum seasonal groundwater levels are likely to be higher later in the wet season.

 

Response to Pumping from Abstraction Wells

Perhaps the first surprise with this data is that there is no obvious response in water levels to the almost continuous pumping from nearby abstraction wells. This is encouraging.

 

Cyclical “Tidal” Changes in Groundwater Level

There is, however, an underlying daily cyclical response in the water level record, with a peak-to-peak cycle of one day and amplitude in the order of 7 to 10 cm. This occurs both in the dry and wet season (Figures 2, 3 and 4).

This does not correlate with ocean tides, which would be expected to produce two cycles per day in the groundwater record, with peaks and troughs moving forward by one hour per day. The times of the peaks in groundwater level (around 5 a.m.) and troughs (around 7 p.m.) were compared to local tide tables by the client, who confirmed there was no correlation between these and the times of ocean tides.

So what causes these fluctuations?

Figure 2: Groundwater Levels and Barometric Pressure (1 Week in March 2018)

Figure 3: Groundwater Levels and Barometric Pressure (1 Week in June 2018)

Figure 4: Groundwater Levels and Barometric Pressure (1 Day in June 2018)

Barometric Pressure Changes

Looking at this data, my first thoughts were that the cyclical changes might bear some relationship to the diurnal gravitational earth tides seen in the barometric record for the site (Figures 1 to 4).

These twice daily variations are very similar to those I’ve recorded in Sierra Leone (See blog Is Barometric Compensation Always Necessary? ). Peaks are observed twice daily (around 10 a.m. and 10 p.m. [Figure 4]) with an amplitude of 4 to 5 mbar (equivalent to 4 to 5 cm of water). However, in the water level record shown above, these effects have already been compensated for, and there is no obvious indication that these pressures have been translated into water level changes in the aquifer.

Other Possibilities

As I was attending a hydrogeological conference the following day, I made a nuisance of myself and asked a few eminent hydrogeologists if they had come across similar phenomenon in their studies. Various suggestions were offered, including responses to abstractions; barometric (earth tide) responses; daily evapo-transpiration losses (mainly during the wet season); and hydraulic loading during recharge (mainly in unconsolidated aquifers in areas of shallow water levels). What was interesting is that single-cycle daily responses were something others have observed in other parts of the Indian sub-continent.

 

Search Engines and Earth Tides

Since abstractions, evapo-transpiration and hydraulic loading do not seem to provide the answer to what appear to be year-round daily cycles in water levels, I thought I’d follow up on earth tides a little further.

Enter Groundwater Levels and Earth Tides into Google and this produces a few notable geophysical references to research going back to the 1960s. Some of these seem to relate solely to confined aquifers (which is not the case here), and most are highly technical. But some papers on unconfined aquifer responses offer what appear to be plausible explanations for the observations in the data.

More on this in my next blog.

 

For more on groundwater level measurements see earlier blogs:

Links to In-Situ level measurement equipment

Follow links here for water level measurement equipment.

 

Peter.Dumble@pdhydrogeology.com

©Peter Dumble 2018
2 December 2018