Figures 8a,b.  Residential monitoring well samples show a positive correlation of d15N with sodium concentration (A) and a negative correlation of d15N with dissolved oxygen saturation (B) which suggests that this heavier nitrogen is associated with septic system wastes rather than fertilization of agricultural fields with manure.
Figures 8c,d.  Public supply well samples show similar trends for both sodium (C) and dissolved oxygen(D). Wells RU1 and DG1 are depleted in dissolved oxygen because they are screened below clay layers.