Decreasing pH values in groundwater

Question

I am writing to you in order to ask the Water network experts for a reliable explanation considering non-expected results in the carbonate equilibrium in the groundwater.
The mean altitude of the research area is 292 m a.s.l. and prevailing land cover types are artificial surfaces (59%) and agricultural areas (39%). The yearly amount of participation is 1454 mm (2019).
The alluvial aquifer is developed in a tectonic basin, filled with highly permeable Quaternary gravel and sand beds, which are partly conglomerated. The thickness of the sedimentary fill is up to 100 m down to its very low permeable pre-Quaternary basement. The groundwater table is on average 25 m below the surface. The unconfined intergranular aquifer with high transmissivity is mainly recharged from the river, the second recharging component is participations.
In the recent decades the decreasing concentration of nitrate and sulphate in the groundwater has been observed as a consequence of agricultural measures.
At the same time the slightly decreasing values of pH has been detected. The changes are hardly detectable and could be overlooked due to insufficient number of analyses per year and various impacts from the environment and anthropogenic activities. The Ca+2 concentration has been recognized  as slowly decreasing too.
My first question is as follows: what are the most probable reasons for decreasing values (toward more acidic values) of pH in groundwater (changes in pC02, decreasing values of NOx and SOx in the air causing the increasing values of pH in the soil (towards the base values!), changes in agricultural practices, changes in recharging mechanisms, etc.). In the average the decrease of 0,1 unit of pH in the groundwater has been observed in the last decade.
The second question goes as follows: One could expect that decreasing concentration of calcium in groundwater would cause the increasing values of pH.  The data show the opposite trend.  Could we have a clear explanation for this phenomenon? Is the surplus of bicarbonate concentration in groundwater together with decreasing concentration of Ca+2 still the driving force for pushing the equilibrium to the right side of the equation:
(HCO3-)aq   ↔  (H+)aq  + (CO32-)aq
(Ca2+)aq   +   (CO32-)aq    ↔   (CaCO3)s
⸏⸏⸏____
(Ca2+)aq   +   (HCO3-)aq    ↔   (H+)aq  + (CaCO3)s
I would appreciate if you could inform me about the similar observations or published articles dealing with these phenomena.
Regards,
Brigita Jamnik

Peter Webb · Answer

Most agricultural fertilisers are acidifying by nature. Perhaps excessive usage and leaching are affecting the water ? A broader analysis of the water may reveal such substances. Regards Peter

Mazhar Ali Rind · Answer

Both reduction of Ca++ ion and pH suggest there is addition of acidic compound in the water most probable cause may CO2 from environment during rain.

Taxonomy