**3.1 Water sampling and hydrogeochemical analysis**

A hydrogeochemical monitoring regime of the Pidong Crater Lake was carried out by sampling the water on a monthly basis to determine any hydrogeochemical changes over the 2 to 3 year study period. The physical parameters of Temperature, pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS) were carried out directly in the field using an MT 806/pH/EC/TDS/Temperature portable meter. The major, trace and rare earth elements analysis (Mg, Ca, Na, K, Cr, Ni, Co, Sc, V, Cu, Pb, Zn, Bi, Cd, Sn, W, Mo, AS, Sb, Rb, Cs, Ba, Sr., Ga, Li, Ta, Nb, Hf, Zr, Y, Th, U, B, Fe, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were carried out using ICP-MS method. The anions (SO4, Cl, HCO3, NO3, F, Br and PO4) concentrations were determined using colorimetric method at Bureau Veritas Minerals Laboratory Ltd., Canada. The oxygen and hydrogen isotopes analyses of the lake, local rainfall and volcanic spring water was carried out at Activation Laboratory Ontario Canada using cavity ring down spectroscopy with V-SMOW as standard and reported in per mill notation (0/00).

The Carbon-14 dating of the lake water was determined by single stage accelerator mass spectroscopy (Source: Activation Laboratories, Canada).
