Groundwater Chemistry on an Island

[TvEp]

Good day all!

I have a question about groundwater chemistry on an island. We have datasets over the last 40+ years and what can be seen is that the eastern and western side of the island differ from each other. On the western side there has been a distinct drop in pH over the last decades, and on the eastern side there is less of a drop in pH, ánd a rise in alkalinity. For both sides of the island the pCO2 is the same. I was wondering if I am going into the right direction when trying to explain what could potentially cause this difference.

East
Drop in pH ↓
No significant difference in HCO3- over the years -
More urban area, with many septic tanks
More nitrogen / ammonium pollution
Same geology as west
Not subjected to events of acid rain
Groundwater wells have same pCO2 as in west (all above atmospheric pCO2)
Same salinity as in east

West
Drop in pH more distinctly ↓↓↓
Rise in HCO3- over the years ↑
More rural area with more vegetation, although less septic tanks still often sampled at people’s houses
Less nitrogen / ammonium pollution
Same geology as east
Might be subjected to some events of acid rain, whereas the eastern side is not
Groundwater wells have same pCO2 as in east (all above atmospheric pCO2)
Same salinity as in west

Question:

• What could be the cause of the more distinct drop in pH on the western side of the island, and how does that relate to the increase in HCO3- on the eastern side of the island?

There is no difference in the pCO2 between the east and the west of the island. I think this says that the influence of CO2 is more or less equal for the eastern and western side, regardless of what the cause of the CO2 or acid influx is on either or both sides of the island (vegetation / septic tanks / acid rain / fertilizer?). All wells (east and west) have a pCO2 that is higher than that of atmospheric pCO2.

Could the less distinct drop in pH on the eastern side of the island be related to the increase in HCO3- that is also seen on the eastern side that is not visible on the western side?

If the acidification on the eastern side is caused primarily by an abundance of leaking septic tanks and/or leaking of (untreated / treated) wastewater from the wastewater treatment facility (that is only present in the more urban east), then there might be an additional source of alkalinity (detergents, or chemicals used in wastewater treatment) in the east which would increase the buffering capacity on the eastern side of the island. But I am not sure it then still fits with the (pCO2) findings?

If it does, then perhaps the acidification in the west could be caused by a combination of different potential processes, such as: vegetation, some events of acid rain, and to some extent septic tank. In the east the pH drop could possibly caused primarily by leaking wastewater, which would mean more relative alkalinity allowing for more buffering, hence the less distinct drop in pH and rise in HCO3-?

Are there some other ratios, etc., that I could look at to get more clarity on this matter?

Would love to get some input!

 

[Borek]

I would check weather patterns, dominating winds, precipitation, clouds etc. Doesn't have to be the culprit, but often is.

 

[jim mcnamara]

As @Borek noted, prevailing wind provides sodium movement into the soil system, assuming a maritime environment. Na is a primary driver of soil pH. So, you should expect a gradient of pH and other other affected results you mention across a cline which is parallel to the prevailing wind.
https://onlinelibrary.wiley.com/doi/epdf/10.1111/ecog.01589

Also how the samples were taken is very important. Sounds like well water tests, i.e., you may have single point samples from varying depths of water wells. In order to compare tests logically, you should compare wells with approximately the same same sampling metrics. Ex: compare all well samples taken at 20m depth.

Islands usually have a dearth of tests. In that case, they are too few and you will have to simply hope that samples you have are representative.

Operational age and withdrawal rates of wells affect adjacent levels of the water tables. This means that as the water table goes down, you may encounter different soil types, with different soil chemistries. Ex: glacial till under an overburden of aeolian (wind) deposits is common in seaside wells above 45 degrees N.

Easy to read reference on the effects of wells on ground water:
https://www.usgs.gov/special-topics/water-science-school/science/groundwater-quality.

This means wastewater can be "sucked" into a well that started out delivering clean water, due to proximity of the well to a wastewater point source.

Do have a soils map of your island? Start with that, add in point source extractions and inputs. After environmental and human inputs, soil types are important sources of cations like K++ and Na+.