The Importance of Quality Monitoring of Groundwater Boreholes Surrounding the Wastewater

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The importance of quality monitoring of groundwater boreholes surrounding the Wastewater Treatment Works in the City of Johannesburg.

Abstract

Johannesburg Water has a network of 64 boreholes established at six wastewater treatment plants in 2002. The study, aims at monitoring the chemical characteristics of groundwater quality, identifying and eliminating possible sources of groundwater contamination caused by our wastewater treatment plants.

Out of 64 boreholes, 12 were chosen from the six wastewater treatment plants; 6 considered as the downstream monitoring and 6 as upstream boreholes. The samples were taken three times monthly, four times yearly, over a period of three years (January 2012 – August 2012) for this purpose.

A number of tests were done in both wet and analytical chemistry laboratories. Samples were analysed for the following determinants, pH; electrical conductivity; ammonia, nitrate ion, alkalinity, chromium, sodium and calcium using pH/conductivity, flow injection analyser and inductively coupled plasma (optical emission/mass spectroscopy) respectively.

The results of six upstream and six downstream boreholes showed a pH range of 5.8 – 7.4, and the ammonia indicated an average of <0.050 mg/L which is comparable to that of drinking water guideline limits. The results also indicated a high level of nitrate in one of the wastewater treatment works boreholes. Both the upstream and the downstream showed a possible contamination of nitrate species. The alkalinity of groundwater for this study differs distinctively ranging from a minimum of 30 – 260 mg/L CaCO3 but relatively comparable between the downstream and the upstream.

Most metals occur naturally in soil and water, however the results for chromium for this study in all samples showed an average reading of <0.010 mg/L. The guideline of 20 mg/L for sodium when exceeded does not require treatment of the water, rather represents a level of sodium in water physicians recommend. Sodium was found to be high from two wastewater treatment boreholes.

KEYWORDS: Quality Monitoring, groundwater, Wastewater treatment works, chemical characteristics

Author: Minah Mere E-mail: Phone: 011728 7373 Fax: 011728 5444

Company: Johannesburg Water SOC Ltd

PO Box 61542

Marshalltown

2107

1. Introduction

Groundwater is the water that is found underground in the cracks and spaces of the soil, sand and rock. Groundwater is stored in, and moves slowly through, layers of the soil, sand and rocks called aquifers. Aquifers typically consist of gravel, sand, sandstone, or fractured rock, like limestone. These materials are permeable because they have large connected spaces that allow water to flow through. The speed at which groundwater flows, depends on the size of the spaces in the soil or rock and how well the spaces are connected.

The area where water fills the aquifer is called the saturated zone. The top of this zone is called the water table as shown in figure 1. The water table may be located only a foot below the ground’s surface or it can sit hundreds of feet down. Groundwater can be found almost everywhere. The water table may be deep or shallow; and may rise or fall depending on many factors. Heavy rains and melting snow may cause the water table to rise, or heavy pumping of groundwater supplies may cause the water table to fall.

Figure 1: Geological survey showing groundwater structure.

Water in aquifers is brought to the surface naturally through a spring or can be discharged into lakes and streams. Groundwater can also be extracted through a well drilled into the aquifer. A well is a pipe in the ground that fills with groundwater. This water can be brought to the surface by a pump. Shallow wells may go dry if the water table falls below of the well. Some wells, called artesian wells, do not need a pump because of natural pressures that force the water up and out of the well.

Objective of the study:

(i)The aim of the study is to monitor the chemical characteristics of the groundwater quality.

(ii)To identify and minimize possible sources of contaminants of groundwater caused by Johannesburg Water treatment plant.

1.1 Background

When rain falls to the ground, the water does not stop moving. Some of them flow along the surface to the streams or lakes, some of it is used by the plants, some evaporates and returns to the atmosphere, and some sinks into the ground. Geochemists have used the term geochemical spheres to describe the various parts of the earth being studied. They include the lithosphere (rocks), pedosphere (soils), biosphere (living organisms), atmosphere (air), hydrosphere (water) and anthroposphere(man’s effect on the spheres). The main processes occurring in these various spheres include the hydrologic cycle, which describes the distribution of water on the planet, and the rock cycle, which describes the distribution of rocks. In addition, when studying pollutant transport and fate, other aspects of the system and the various interactions between various spheres must be considered.

Minerals dissolve and contribute to water quality, as do several common gases –CO2, which affects the pH of water, and H2S and O2 which often determine the redox of water. The presence and amount of clay minerals, amorphous oxides, and natural organic matter exert a strong influence on the mobility or retardation of both metals and synthetic organic pollutants in the groundwater system.

1.2 Pollutants in groundwater

The geological nature of the soil determines the chemical composition of groundwater. Water, being an excellent solvent, can dissolve gases, liquids and solids and thereby increase concentrations of solutes in groundwater.

1.2.1 Nitrogen Species (Ammonia and nitrates)

Nitrogen is a major constituent of the earth’s atmosphere and occurs in different gaseous forms such as elemental nitrogen, nitrate and ammonia. Natural reactions of atmospheric forms of nitrogen with rainwater result in the formation of nitrate and ammonium ions. Nitrates (NO3-) occurs in almost all natural waters. Concentrations range up to hundreds of mg/L except when contamination is present; they seldom exceed 20 mg/L. However 10 mg/L Nitrate-nitrogen or greater may be regarded as a probable indication of contamination from fertilizers, municipal wastewaters, feedlots, septic systems, and sometimes the cultivation of grasslands. Of the common nitrogen species the nitrate ion is not readily absorbed by clay minerals. It moves freely through the aquifer, which is in contrast to ammonium ions that are strongly adsorbed by some clay minerals.

The primary source of all the nitrates is atmospheric nitrogen gas. This is converted to organic nitrogen by some plant species by a process called nitrogen fixation. On the death of the plants the organic compounds are decomposed by microorganisms to inorganic ammonium salts (ammonification). These in turn are converted to nitrates by a process called nitrification. In environments that are depleted in oxygen, some microorganisms can use nitrate in place of gaseous oxygen to carry out their metabolic processes. The products of this reaction are nitrogen gas and nitrous oxide (N2O). This process, called denitrification, effectively removes nitrogen from the subsurface. pH may also be a critical parameter in these processes. Ammonia will only occur in very reduced waters where H2S and CH4 may also be present.

1.2.2 Alkalinity

The pH of groundwater controls which type of carbonate occurs in solution. In acidic solutions, H2CO3 is the dominant carbonate anion, followed by HCO3-, then CO32- as solutions become more basic. Water that is a good buffer contains compounds such as bicarbonates, carbonates and hydroxides, which combine with H+ ions from water thereby raising the pH (more basic) of the water. Alkalinity is a measure of the ability of a water sample to neutralize acids, thereby maintaining a fairly stable pH. As the concentration of CaCO3 increases, the alkalinity increases and the risk of acidification decreases.

Alkalinity comes from rocks and soil, salts, certain plant activities and certain industrial wastewater discharge (detergents and soap based products are alkaline). If an area’s geology contains large quantities of calcium carbonate (CaCO3, limestone), water bodies tend to be more alkaline.

1.2.3pH

Acid-base reactions can result increases and decreases of protons (i.e. hydrogen ions).pH measures the concentration of H+ and hydroxide (OH-) ions which make up water (H2O). When the two ions are in equal concentrations, the water is neutral, whereas the water is acidic if H+isgreater than OH- and basic when OH- is greater than H+.

pH= -log [H+]

The pH of groundwater controls which cations, anions, gases and solids dissolve into ground water solution and which exit from groundwater through precipitate or/and volatilize. pH is one of the most important parameter affecting the chemical composition of groundwater. Anything that changes the pH of the sample will likely affect other constituents as well. More acidic pH of groundwater is capable of dissolving minerals. Halides, sulfate, carbonate minerals dissolve quite readily, over relatively short period of time, in acidic ground water. Metals also tend to be mobilized by complexes at low pH. This is effective in mobilizing iron, mercury and radium for example. The presence of acid rain can lower the pH of surface water then groundwater. Rain unaffected by man’s activities will have a pH of 5.5-6.5 because of dissolved CO2. Minor quantities of nitric acid may be present because of the oxidation of N2 by lightning and forest fires.

1.2.4 Electrical conductivity

The conductivity of water depends on the presence of ions, their total concentration, mobility, valence and relative concentrations, and on the temperature of measurement. Solutions of most inorganic acids, bases and salts are relatively good conductors. Conversely, molecules of organic compounds that do not dissociate in aqueous solutions are poor conductors.

Conductivity is the ability of an aqueous solution to conduct an electrical current. The electric conductivity (EC) of water is measured as the reciprocal of the resistance measured between two parallel metals plates through an aqueous solution at a specified temperature. Conductivity gives a good indication of total dissolved solids (TDS). The relationship between TDS and EC for most groundwater is linear.

1.2.5 Arsenic

Arsenic can be found naturally on earth in small concentrations. It occurs in soil and minerals and it may enter air, water and land through wind-blown dust and water run-off. Arsenic in the atmosphere comes from various sources; volcanoes release about 3000 tonnes per year and human activity is responsible for much more 80.000 tonnes of arsenic per year are released by burning of fossil fuels. Despite its notoriety as a deadly poison, arsenic is an essential trace element for some animals, and even for humans, although the necessary intake may be as low as 0.01mg/day.

Due to human activities, mainly through mining and melting, naturally immobile arsenics have also mobilized and can now be found on many more places than where they existed naturally. Arsenic is one of the most toxic elements that can be found. Despite their toxic effect, inorganic arsenic bonds occur on earth naturally in small amounts. Humans may be exposed to arsenic through food, water and air. Exposure may also occur though skin contact with soil or water that contains arsenic.

Arsenic exposure may be higher for people who work with it, who live in houses that contain conserved wood of any kind and for those who live infarmlands where arsenic-containing pesticides have been applied in the past. Exposure to inorganic arsenic can cause irritationof the stomach and intestine, decreased production of redand white blood cells, skin changes and lung irritation.

1.2.6 Calcium and magnesium

Water that contains a lot of calcium and magnesium is said to be hard. The hardness of water is expressed in terms of the amount of calcium carbonate- the principal constituent of limestone or equivalent minerals that would be formed if the water were evaporated. Water is considered soft if it contains 0 to 60 mg/L of hardness, moderately hard from 61 to 120mg/L, hard between 121 and 180 mg/L, and very hard if more than 180 mg/L. Very hard water is not desirable for many domestic uses; it will leave a scaly deposit on the inside of pipes, boilers, and tanks.

1.2.7Chromium

Chromium (Cr) is a metal found in ores, soils and plants. Chromium compounds from natural sources are usually found in groundwater in trace amounts only. The most common man-made sources of chromium in groundwater are

• Burning of fossil fuels
• Mining effluents
• Effluents from metallurgical, chemical and other industrial operations.

Chromium may affect the taste or smell of well water, but not at levels normally found in groundwater. The Canadian drinking water quality guideline for chromium is 0.05 milligrams per litre (mg/L).

Chromium can be present in water in two forms, trivalent chromium and hexavalent chromium. Chromium 3 and chromium 6 have very different toxicity characteristics. Chromium 3 is more commonly found in water. Chromium 3 is essential to human nutrition and considered non-toxic. When chlorine is present, chromium 3 turns into chromium 6.

Exposure to chromium 6 at levels above 0.005 mg/L in drinking water may cause diarrhoea, vomiting, abdominal pain, indigestion, convulsions, and liver, kidney damage.

1.2.8 Sodium

Sodium is a highly soluble chemical element with the symbol “Na”. Sodium is often naturally found in groundwater. In water, sodium has no smell but it can be tasted by most people at concentrations of 200 milligrams per litre (mg/L) or more. High concentrations of sodium in groundwater occur naturally in some areas. For example, on the Gulf Islands sodium levels have been shown to range up to thousands mg/L depending upon the location and depth of the well. An increase in sodium in groundwater above ambient or natural levels may indicate pollution from point or non-point sources or salt water intrusion.

All groundwater contains some sodium because most rocks and soils contain sodium compounds from which sodium is easily dissolved. The most common sources of elevated sodium levels in groundwater are as follows:

• Erosion of salt deposits and sodium bearing rocks minerals
• Naturally occurring brackish water of some aquifers
• Infiltration of surface water contamination by road salt
• Irrigation and precipitation leaching through soils high sodium
• Groundwater pollution by sewage effluents
• Infiltration of leachate from landfills or industrial effluents.

Sodium is a principal chemical in bodily fluids. It is not considered harmful at normal levels of intake from combined food and drinking water sources. However, increase intake of sodium in drinking water may be problematic for people with hyper tension, heart disease and kidney.

2. Materials and methods

2.1 Study area

Groundwater samples were collected from boreholes (n=12) located in the region 1 to 6 (Driefontein, Bushkoppie, Olifantsflei, Northern Works, Goudkoppies and Ennerdale) in Johannesburg Water treatment plants. Six are considered as the downstream monitoring and six as the upstream boreholes. The sampling was carried out over 3 year’s period.

2.2 Collection of samples

The purging and sampling of borehole samples was done by an outsourced company because of specialized equipment needed. Samples were collected in plastic bottles and weremeeting the requirements of the sampling program. They were handled so that those are not deteriorated, contaminated or compromised before analysis.

2.3 Sample analysis

A number of tests were done in both wet and analytical chemistry laboratories but few were chosen for this paper covering the electrochemistry and inorganic chemistry (major and minor cations and anions determinants). Samples were analysed for pH using pH meter, electrical conductivity using conductivity meter, anions and cations (NH3, NO3, alkalinity) using the Flow Injection Analyser and the macro and micro elements (As, Cr, Na, Mg, Ca) using Inductively Coupled Plasma (optical emission spectroscopy/ mass spectrometry).

2.3.1 pH and electrical conductivity

Potentiometric method was used for both pH and EC measurements. pH meter consist of a potentiometer, a glass electrode and temperature-compensating device while conductivity meter has a commercial probe containing a temperature sensor. A test solution of about 100 mL was poured into each beaker. The measurement was taken for both pH and conductivity and temperature noted.

2.3.2 Ammonia, nitrate and alkalinity on flow injection analyser.

Ammonia was determined based on Berthelot reaction. Ammonia reacted with alkaline phenol then sodium hypochlorite to form indophenol blue. Sodium nitroprusside was added to enhance sensitivity. The absorbance of the reaction product was measured at 630nm, which is directly proportional to the ammonia concentration in water.

Nitrate was quantitatively reduced to nitrite by passage of sample through a copperized cadmium column. The reduced nitrate plus original nitrite were determined by diazotizing with sulphanilamide followed by coupling with N-(1-naphthyl) ethylenediamide dihydrochloride. The resulting color was read at 520 nm.

The sample was injected, the poorly buffered methyl orange changed color in proportion to the change in pH of the weak buffer, and thus in proportion to the alkalinity of the test sample.

2.3.3 Calcium, magnesium, sodium, chromium and arsenic on ICP.

The sample was transported into the nebulizer as a stream of liquid, then converted into aerosol. The aerosol was transported to the plasma where it was desolvated, vaporized, atomized and ionized. The excited atoms and ions emitted their characteristics radiation which was collected by a device that sorted the radiation by wavelength. The radiation was detected and turned into electromagnetic signals which were converted into concentrations.

3. Quality assurance and quality control program

To assess the precision and accuracy of the results standards, blanks, quality assurance solutions were done for each analyte. Correlation coefficients were determined to find the accuracy of the instruments.

4. Results and discussion

The results below in table 1and 2 were performed with groundwater obtained from different specially drilled boreholes surrounding six Johannesburg wastewater treatment plants. These boreholes were continuously monitored for the following tests for this purpose: pH, Conductivity, Alkalinity, Ammonia, Nitrate and metals which are Calcium, Magnesium, Sodium, Arsenic, and Chromium.

Table 1: Chemical characteristics (pH, EC, alkalinity, ammonia, and nitrate) of groundwater samples.All results in (mg/L) except pH and EC (mS/m).