**Abstract**

The occurrence and recurrence of floating 'mat' vegetation around the Pidong Crater Lake and intermittent change in the color of the lake water from natural blue to brown-yellowish color call for a better understanding of the factors controlling these biological and physico-chemical changes. As a result of these changes, a hydrogeochemical monitoring regime was employed where the lake water samples were collected and analyzed on a monthly basis over a period spanning for 3 years. The hydrogeochemical results for the lake water when plotted in the piper trilinear diagram display predominantly Mg-Ca-HCO3 type with minor water types of Na-K-HCO3, Na-K-Cl and Na-K-SO4 types (≤ 20%), suggesting a derivation from water-rock interaction processes (hydrolysis). The visible dominance of the alkali earth metals (Mg2+ + Ca2+) over the alkali metals (Na+ + K+ ) is expected from a basaltic host rock aquifer due to the dissolution of its constituent ferromagnesian minerals and plagioclase respectively. The general depletion of LILE (Ba, Rb, Sr) (1–10 x Chondrite) and that of High Strength Elements (HSE) (Nb-Yb) (0.001 to <1 Chondrite values) suggest that these elements are from sources (basaltic rocks) highly depleted in these elements. Study of Oxygen-18 (δ 18O) and Deuterium (δ<sup>2</sup> H) study of the lake water shows that it is of meteoric origin and of relatively recent age (230 ± 30 yrs. before present). The increase in PCO2 in the lake water triggers the increase in Fe concentration (335 ppb to 429 ppb) which is manifested by the sporadic color change and increased acidity in the lake from pH 7.35 to pH < 6.71.

**Keywords:** Pidong Crater Lake, Jos Plateau, hydrogeochemistry, water color changes, fumaroles (CO2), meteoric water
