Please check the link given below.

http://www.omicsonline.org/roles-of-isotopes-recharge-analysis-in-studying-water-in-fractured-aquifer-under-climatic-variability-2157-7617.1000148.php?aid=18514

Hi, many processes may play a role, but the most important are the infiltration of rain (with different isotope composition) , mixing of different types of waters and of course the amount of evaporation. So if your shallow groundwater is in an area where part of it is exposed in lakes  and allowed to evaporate- that may explain the enrichment in heavy isotopes.

I found the manuals by the atomic agency in Vienna very helpful to learn about isotopes. Here is the link to the first volume, but there are many more

http://www-naweb.iaea.org/napc/ih/documents/global_cycle/Environmental%20Isotopes%20in%20the%20Hydrological%20Cycle%20Vol%201.pdf

Also this paper and refs in there, is very nice:

Skrzypek, G., A. Mydłowski, S. Dogramaci, P. Hedley, J. J.

Gibson, P. F. Grierson. 2015. Estimation of evaporative

loss based on the stable isotope composition of water

using Hydrocalculator Journal of Hydrology. 523: 781–

789. doi:10.1016/j.jhydrol.2015.02.010

And if you like to compare with another case study using stable isotopes, see these papers that you can both download from my researchgate page.  We looked at groundwater and waterbudgets of gravel pit lakes , one in fresh water setting and one in coastal brackish water setting.

good luck - Pauline

Mollema, P.N., Stuyfzand, P.J., Juhasz-Holterman, M.H.A., Van Diepenbeek, P.M.J.A.,

Antonellini, M., 2015a. Metal accumulation in an artificially recharged gravel pit

lake used for drinking water supply. J. Geochem. Explor. 150, 35–51. http://dx.doi.

org/10.1016/j.gexplo.2014.12.004.

Mollema, P.N., Antonellini, M., Dinelli, E., Greggio, N., Stuyfzand, P.J., 2015b. The influence

of flow-through saline gravel pit lakes on the hydrologic budget and hydrochemistry

of a Mediterranean drainage basin. Limnol. Oceanogr. 60, 2009–2025. http://dx.doi.

org/10.1002/lno.10147.

I think you may find useful hints for interpreting your data in the following book, pages 132 to 135, as there are a couple of figures showing regression lines with slopes less pronounced than that of the GMWL.

Isotopes in the Water Cycle -- Past, Present and Future of a Developing Science

Aggarwal, Gat & Froehlich, Springer, 2005.

I found a "free" web access to these pages via Google Books (see the link below).

https://goo.gl/RLec6p

 thank you for the question. I feel that this question is on seismicity.  when there is a deviation in slope, it concerns the velocity as well. from my understanding i think it is due to the fact that the groundwater is polluted or contaminated