**5.1 Sources of major ions**

The Gibbs diagram for the Pidong Crater Lake shows that the lake water is characterized mainly by water-rock interaction processes [14]. The hydrogeochemical abundance order of the lake (Mg2+ > Ca2+ > Na+ > K+ ) and that of the host rock (MgO > CaO > NaO > K2O) show similarity and may suggests derivation from the overlaying volcanic rocks. Similarities in hydrogeochemical spectral patterns of lake and host volcanic rocks are indicators of sources of derivation [4, 15, 16]. The major cations of groundwater are derived mainly from chemical weathering [17]. Generally, the silicate minerals present in the basalts, pyroclastic materials, ash and pulverized granite get weathered by hydrolysis producing mainly Mg-HCO3 and Na+ and K+ depleted groundwater. Groundwater containing CO2 2− can easily react with orthoclase to release Magnesium ion, Calcium bicarbonate ions (Mg2+, Ca2+, HCO3 − ) [18]. The Na+ and K+ alkali appear to originate from hydrolysis of plagioclase in the basalt and granite rocks. In water-rock interaction processes the following chemical reaction of volcanic rocks may occur and hence the chemical sources are derivable from these processes.

1.Hydrolysis of plagioclase minerals

$$\text{CaAl}\_2\text{SiO}\_8 + \text{HCO}\_2 + \text{l/2}\,\text{O}\_{2\to}\text{Al}\_2\text{Si}\_2\text{4(OH)} + \text{CO}\_3^{2-} \tag{1}$$

2.Oxidation (chemical weathering) for ferromagnesium minerals (Pyroxene)

$$\left(\text{Mg,Fe,Ca}\right)\text{SiO}\_3\text{9H}\_2\text{CO}\_3 \rightarrow \left(\text{Mg}^{2+}, \text{Fe}^{2+}, \text{Ca}^{2+}\right) + 4\text{HCO}\_3^- + \text{HCO}\_3 + \text{H}\_4\text{SO}\_4 \tag{2}$$

#### **5.2 Trace elements concentration**

It is observed that Fe concentration have significant increase by 200% from 0.27 mg/l in 1986 to 0.612 mg/l in present study [12]. Lake water, Fe concentration gradually increases from 423 ppb in November, 2014 to 3249 ppb in March 2015 during the lake color change activities. This phenomenon is similar to the Lake Nyos CO2 gas disaster episode of 1986 [19, 20]. The lake color change phenomenon was attributed to upliftment of deep anoxic water carried upwards to shallow lake level where oxidation of ferrous iron precipitates to form murky water.

The spider plot for the LILE normalized to upper crustal values (UC) (**Figure 2**) displays strong positive anomalies in Sr. (>1x UC values), Ba (>0.1x UC values) and enrichment in Rb (0.1x UC values). There is a visible negative anomaly in Zr (0.0001 x UC values), all together suggesting that these elements are originally from a source that is highly depleted in these elements (probably deep mantle source) [21]. The dominant rock types that comprise of the crater lake/maar are basalts, pyroclastics and volcanic ash. Generally, hydrogeochemical constituents of any groundwater are signatures of the background rock types the water precolates through [17, 22].

### **5.3 Rare earth elements concentrations**

Generally, REE spidergraph of the Pidong lake water normalized to chondrite values show high impoverishment or depletion (< 0.01 to <1 X chondrite) (**Figure 4**). The slight enrichment in the LREE (La-Sm) is suggestive of influence by crustal rocks materials (host granitic basement rock) rich in these elements (**Figure 4**).

There is high similarity in the REE patterns in the well waters compared to that of the lake and the spring water. The well waters generally exhibit higher REE concentrations (. > 1x Chondrites) but maintain similar LREE enrichment relative to HREE suggestive of significant influence from the surrounding basement complex host rocks. The impoverishment in REE in the lake and spring water suggests a derivation from a mantle source depleted in these elements [4, 15, 16, 22].

#### **5.4 Anion concentrations**

The concentration of the SO4, Cl and F are generally influenced by the waterrock interaction processes between rainfall and host volcanic/granitic rocks. Generally, HCO3 − is derived from dissolution of silicate minerals of orthoclase, plagioclase, hornblende, diopsite, olivine and biotite of country rock by carbonic acid [18, 23–25]. A comparative analysis of the hydrogeochemical anion concentrations of the lake in this study with that of Patterson [9] show that there is significant increase in Cl, SO4 and Fe and corresponding decrease in alkalinity and pH over the last 29 years. These increases have been attributed to intermittent CO2 degassing (fumaroles) into the lake from the subsurface hydrothermal system [12].

#### **5.5 Origin and age of water**

The δ 18O and δ<sup>2</sup> H compositional values for the lake (**Table 2**) falls within meteoric water composition values [12, 26] and this is further affirmed by the plot (**Figure 5**) of the SMOW line in the δ 18O and δ<sup>2</sup> H correlation diagram [26–28]. The hydrocarbon-14 (14C) age of the lake water is calculated to be 230 ± 30 yrs. and this affirm, that the lake water is relatively young and suggests constant replenishment by precipitation.

#### **5.6 Lake color and hydrogeochemical changes**

The Pidong Lake color change activities are characterized by decrease in pH values 7.39 to 6.71, 7.15 to 6.83 and 7.18 to 6.20 as observed in November 2014, October 2015 and September 2016 respectively. The intermittent lake color changes associated with CO2 degassing into the lake and its dissolution forming weak acid. The HCO3 − specie increases over time and resulting in decrease in pH from 9.35 to neutral or slightly acidic as seen in the comparative study (**Table 1**). This process can be explained as follows:

CO2 (gas), CO2 (aq), HCO3, CO3 2−, specie are related to pH and temperature of fluid [28]. When CO2 (gas) undergoes hydration and ionization, hydration of CO2 forms H2 − CO3 2−, CO2 gas dominated sub aqueous relevant with the lake

$$2\text{H}\_{\text{z}}\text{O} + 2\text{CO}\_{\text{z}} \rightarrow 2\text{H}\_{\text{z}}\text{CO}\_{\text{3}}\tag{3}$$

$$\text{Ionization process, } \mathsf{2H}\_{\mathsf{z}} \mathsf{CO}\_{3} \to \mathsf{2H}\_{\mathsf{z}} \text{ }^{\ast}\_{\cdot \cdot} \mathsf{CO}\_{3} \,^{2-} + \mathsf{CO}\_{3} \,^{2-}. \tag{4}$$

It is observed that the highest ionization correlates with the lowest pH (6.71) during the period of lake color change. This result suggests a magmatic gas/fumaroles (CO2) connecting upwards below the lake from shallow mantle and mixing with shallow groundwater and controlled by meteoric recharge. This similar phenomenon was observed in Popocapetl volcanic springs [28]. The intermittent CO2 gas and associated gas input into the lake during color change activities over time is *Hydrogeochemistry of the Pidong Crater Lake, Jos Plateau Volcanic Province, Nigeria… DOI: http://dx.doi.org/10.5772/intechopen.99720*

suggestive of the cause of the decrease in pH from 9.35 to neutral or near acidic as well as alkalinity from 330 mg/l to 147 mg/l [9] as compared with present study.
