Hydrogeochemical Processes Influencing Groundwater Quality in the Lower Pra

Basin, Ghana.

A PhD ThesisBy

Collins Tay

Components of the Thesis

The Thesis is made of 4 parts:

1. Sources of dissolved ions in GW using; GW geochemistry,

Major ion relationships, and stable isotopes of 18O and 2H

2. Using the concept of differences in (TDS), to categorize waters into different Gps and SGps of

similar WQ characteristics (HCA )

3. Using multivariate statistical methods to facilitate the unveiling of hidden structures in the datasets and

assist in resolving key geo-environmental problems .

4. Heavy metal pollution in GW within the basin

Origin of dissolved ions in GW using major ions relationship

INTRODUCTIONGW remains the most dependable source of water supply for the numerous

scattered rural communities in Ghana (including the Pra Basin) due to the contamination of surface water resources.

Knowledge of the chemistry of GW in these communities therefore, need to be well documented for the development and management of the water resources.

Interest in the geochemistry of GW has increased during the past decade as evidenced by hydrogeochemical studies which are becoming a firm part of

regional hydrogeological studies.

Hydrogeochemistry, seeks to determine the origin of the chemical composition of GW and the relationship between water- rock -chemistry,

particularly as it relate to GW movement.

The geochemistry of GW is complex due to infiltration of water through complex geological formations and human impact on flow systems

Introduction cont.

Water being a good solvent, dissolves chemical constituents from dissolvable materials as it passes through the atmosphere, soil and unsaturated zone into the water-table and in doing so, becomes modified in composition

These changes reflect the signatures of factors such as; soil/rock composition,

prevailing climatic condition, pH,

the resident time of water,and topography

The overall implication of these changes is that the hydrochemical facies (WATER TYPES) of GW change in

response to its flow path and geochemical history

Introduction cont.

Investigations into the origin of dissolved ions in water resources

reveals a great deal of information such as:

general geology, degree of chemical weathering quality of recharge water, and inputs from sources other than

water-rock interaction.

Problem statement

The Basin like any other rural community in Ghana, depend heavily on GW as a major source of water supply due to contamination of SW as a result of farming and/or mining activities.Exposure of sulphide rocks (pyrite and arsenopyrites) that contain gold ore, to the atmosphere often results in acid mine drainage generation and subsequent mobilisation of trace metals, particularly, As in high proportions into the GW systemThe indiscriminate use of mercury (Hg) and other chemicals during mining operations, Despite the susceptibility of GW within the basin to pollution, information on detailed hydrochemical data ensuing to the explicit understanding of GW chemistry is either very little or non existent. Therefore, the need to evaluate the natural geochemical and biochemical processes as well as anthropogenic activities that influence the suitability of GW for drinking cannot be over emphasised.

Objectives

Determined the origin of dissolved ions in GW within the basin using the Mass-Balance approach (major ions-relationship and equivalent ratios of

ions)

Methodology

Sampling and analysis

Sampling was carried out once every 3 months for two years. The reconnaissance sampling was carried out in March 2011 and the last batch of samples were collected in March 2013.

Sampling protocol described by Classen (1982), Barcelona et al., (1985) were observed.

Samples were analyzed using appropriate certified and accepetable international procedures outlined in Standard Methods for the Examination of Water and Wastewater (APHA, 1998)

Charge balances were calculated using the eqn. below

CB = [(∑z Mc - ∑z Ma) / (∑z Mc + ∑z Ma)] * 100

Location of the Lower Pra Basin

The Lower Pra Basin is located between longitude 05o 0'0"N and Longitude 06o 0'0"N and Latitude 01o0'0"W and Latitude 02o 0'0"W (Fig 1)

 

The Lower Pra Basin and the sampling communities

Results and Discussions

Von Bromssen (1989), Caritat (1995)

-14.5 -14.0 -13.5 -13.0 -12.5 -12.0 -11.5 -11.0 -10.5 -10.0-2.00.02.04.06.08.0

10.012.014.016.018.0

ANC NANC

Aci (meq/L)

AN

C/N

AN

C (

meq

/L)

Stumm (1992); Caritat et al., (1998)

Results and Discussions

Maclean and Jakowski

(2000)

Maclean and Jakowski

(2000)However, HCO3/SiO2 < 5 (for 75% of GW) suggesting Silicate

weathering

Maclean and Jakowski (2000)

Brady and Walter (1989); Appelo and Postma (1999)

Results and Discussions

CaAl2Si2O8 + 2CO2 + 3H2O Al2Si2O5(OH)4 + Ca2+ (Anorthite) (Kaolinite) + 2HCO3

However, Na+/ (Na+ + Cl-) > 0.5 for 81 % of GW, suggesting ion - exchange reactions responsible for Na+ and Cl-

Results and Discussions

Similarly, the SiO2/(Na++K++Cl-) < 1 for 100% of GW, suggesting cation ion exchange reactions

Results and Discussons

2NaAlSi2O8 + 2CO2 + 11H2O = Al2Si2O5(OH)4 + 2Na+ + 2HCO3- +

(Albite) (Kaolinite) 4H4SiO4

Conclusions

The major processes responsible for chemical evolution of GW within the basin include: silicate (SiO4)4- weathering and ion-exchange reactions, sea aerosol spray, oxidation of pyrite (FeS2) and arsenopyrite (FeAsS).

The GW is moderately acidic to neutral (3.5 - 7.0 pH units). Approximately, 89% of GWs had pH values outside the WHO (2004) Guideline Value due principally to natural biogeochemical processes.

The relative abundance of cations and anions is in the order: Na+ > Ca2+> Mg2+ >K+ and HCO3

- > Cl- > SO42- respectively.

Recommendations

In terms of management and development of GW, future GW studies could be focused on similar hydrogeochemical

studies in the Upper Pra Basin to generate similar data.

Data obtained from the hydrogeochemical studies in the Lower and Upper Pra Basins could be put together and the

results obtained used to model the hydrogeochemical processes influencing the quality of GW within the Pra Basin.

The model could then be used to predict GW quality for

decades to come thereby, providing the basis for the monitoring of water quality parameters to ensure the efficient

management and development of GW within the Pra Basin

THANK YOU FOR

YOUR ATTENTIO

N