Evaluation of Residual Chlorine Concentration in the Water Distribution Networks: An Analytical Study

2.1. Explanation of the Studied Area

The central Baquba water treatment plant is situated in the Al-Sawamarah area of Baquba, northwest of the city, and pumps drinking water to all regions of Baquba city. The water treatment plant started production in 1985. The design capacity of the water treatment plant is 75 million gallons/day, served by five ground tanks spread throughout the city. This project feeds about 450,000 people in the city of Baquba. This would signify the importance of the current research. The key source of water is the Khuraisan River, and the water is treated in the water treatment plant using chlorine gas. Five lines were selected from the pipes feeding the liquefaction network, as indicated in Table 1 and Fig. (1), representing the different sides of the areas fed by the central Baquba water project. Three points were selected along each line to collect and examine water samples every week throughout the study period from November 2022 to May 2023. The 90 water samples were collected from the sites using 2-liter plastic bottles after washing them twice with the sample water at each station. The plastic bottles were well sealed and transported to the laboratory for the required physical tests. All examinations were conducted in the central Baquba water project laboratory.

2.2. Water Quality Standards and Compliance Criteria

To certify a standardized evaluation of water quality throughout the Baquba distribution network, the projected parameters were contrasted with local and international benchmarks. Referring to the Iraqi drinking water standard (IQS 417) and World Health Organization (WHO) guidelines, Table 2 depicts the limit values of residual chlorine, turbidity, and associated microbial indicators. These limits serve as the chief basis to determine water quality.

Based on Table 2, a residual chlorine concentration below 0.5 ppm in tap water is considered non-compliant, while turbidity beyond 5 NTU is categorized as a breach of local and international safety limits.

2.3. Sampling and Water Analysis

Ninety water samples were gathered from the drinking water network of the Baquba Central Project in several regions during the 7 months from November 2022 to May 2023. The physical examinations were conducted in the laboratory of the Baquba Central Water Project. Water samples were analysed to evaluate key physicochemical and biological parameters, including pH, electrical conductivity (EC), residual chlorine concentration, turbidity, and total coliform bacteria. Water analyses were achieved utilising the updated standard methods approved by the American Public Health Association (APHA) (APHA, 2005), ensuring high precision and compliance with international laboratory standards. The residual chlorine concentration was measured utilising a calibrated color comparator (Hach, USA). The device has a detection limit of 0.1 ppm. To guarantee the accuracy and reliability of the associated findings, the comparator was calibrated according to the manufacturer's instructions utilising standard chlorine solutions. Also, all measurements were conducted in duplicate to certify reproducibility, and the average values were considered for further analysis. The full descriptions of these examination methods are provided in the following subsections.

2.4. GIS Modeling (IDW Technique)

GIS techniques can enable the integration and analysis of both spatial and non-spatial data at a given location (Madhu et al., 2023). This technique can be used to estimate water quality parameters at different locations where direct measurements are lacking, affording clear insights into the overall water quality within the studied area (Jha et al., 2020).

GIS has been established as a priceless resource for generating spatial distribution maps. This technique is indispensable across different domains, including engineering and environmental science, as it enables effective storage, visualization, and analysis of spatial information. The obtained maps can successfully demonstrate variations in concentrations of different chemical factors. Specifically, the Inverse Distance Weighting (IDW) interpolation technique in the ArcGIS Pro Spatial Analyst module was used to generate these distribution maps and thereby visualize variation in chlorine concentrations (Masoudi, 2021).

Following the methodological approach of Singh and Verma (2019), the current research used ArcGIS (version 10.8) for spatial interpolation, utilising the Sentinel-2 satellite image as the base map for the investigated area. To optimise this model, several power parameter values (p) were systematically evaluated based on cross-validation results. This analysis indicated that p = 4 yields the lowest root mean square error (RMSE) among the tested values. This, in turn, indicated high prediction precision and a minimum margin of error between measured and predicted values. Thus, p = 4 was approved for the final spatial analysis. Also, comprehensive cross-validation findings, including Mean Absolute Error (MAE) and RMSE, have been included in this research to confirm the consistency of the chlorine distribution maps.

3.1. Water Temperature

The temperature of the water samples was measured in degrees Celsius, and the results are presented in Fig. (2). The water temperature ranges from 40°C, the highest recorded in the fifth line in June, to 10°C, the lowest recorded in the first line in March. A usual trend of a gradual increase in water temperature can be detected from November 2022 to May 2023. The fluctuated temperature between the selected five lines indicates a significant seasonal effect on water temperature variation. This fluctuation can undoubtedly affect the metabolic, physiological, and geochemical processes in the river water (Al-Obaidi et al., 2025).

3.2. pH

The average pH results are shown in Fig. (3) and fall within the range of 6.8 to 7.8. This indicates a slightly alkaline nature of the collected water samples, which can be attributed to the limestone character of the Tigris River as it passes through several mountainous areas. More importantly, the pH values are within the WHO and Iraqi drinking water standards, which range from 6.5 to 8.5 (Khadim and Oleiwi, 2021). In particular, clarification and disinfection steps require controlling the pH to optimise the effectiveness of the water treatment. In this regard, the WHO recommends 8.0 to guarantee effective disinfection using chlorine.

3.3. Turbidity

Turbidity is caused by various materials, including organic and inorganic matter, fine particles of soil and sand, and microorganisms. This can be expressed through the visual properties of light scattering or absorption rather than moving in a straight line. Turbidity is a measurement of water transparency for suspended solids and high levels of pathogenic organisms. High turbidity levels can shield living organisms from disinfection, so the treatment and disinfection processes must be effective enough to maintain turbidity at a minimum level. Based on the obtained results, the estimated turbidity values range from 4 to 50 NTU, as depicted in Fig. (4). These results are not within the global standards for drinking water. The high turbidity levels in the water samples during rainy seasons are attributed to mud, soil erosion, and suspended matter, including increased rainfall-driven organic matter and algae, flowing toward the river. This leads to increased turbidity in winter due to increased water level and movement, which prevents the settling of suspended matter. The lowest turbidity reading was recorded in the first line in November, while the highest turbidity reading of 50 was recorded in January, which was characterized by increased rainfall for the year 2023-2022.

3.4. Electrical Conductivity

Electrical conductivity (E.C.) is an arithmetic expression of the negative and positive ions present in water. The E.C. of water is primarily contingent on the electrolytes, which are substances capable of being dissolved. Its value is affected by the concentrations of salt ions. The electrical conductivity of water ranged between 568 and 980 micro-Siemens per centimeter (μS/cm), as depicted in Fig. (5). The increase in the electrical conductivity (positively related to the concentration of dissolved salts in river water) coincides with seasonal variations, with higher values observed in winter compared to summer due to rainfall (Oleiwi et al., 2025). However, in the current study, the highest recorded reading is 980 μS/cm in April 2023, as the rainy season extended until May, resulting in increased river flow and a higher proportion of dissolved salts and consequently the electrical conductivity. The dissolved salts in water lead to a large amount of highly soluble carbon dioxide, which increases the concentration of negatively charged carbonate ions that associate with positively charged ions, forming non-homogeneous salts. All sample results throughout the study period complied with the Iraqi Standard No. 417 of 1989 and the WHO guidelines of 1993 (Galal-Gorchev, Ozolins, and Bonnefoy, 1993), which set the suitability of drinking water based on its E.C. not exceeding 2000 (μS/cm).

3.5. Residual Chlorine

The key objective of having chlorine in water treatment plants is to ensure disinfected water and to exclude microorganisms that can cause disease. To ensure effective disinfection, chlorine is added in a sufficient amount. Chlorine gas is specifically added to the water treatment process after the filtration processes to kill microbes and ensure sterilization, and to eliminate a significant amount of organic compounds that can accompany chlorine and form toxic and carcinogenic substances. In this aspect, the chlorine should remain to prevent biological contamination of water during storage and transportation in the distribution network. The allowable free chlorine concentration at user points should be between 0.2 and 0.5 ppm (Geldreich, 2020). If the residual chlorine decreases, the likelihood of microbial contamination of water increases, especially in the event of leakage or malfunction in the water network. On the other hand, if chlorine levels increase, it can be dangerous due to the formation of compounds with organic substances that can have health effects. More importantly, the residual free chlorine after the water treatment plant disinfection process (plant outlet free-chlorine concentration) should be between 2.5 and 5 ppm in drinking water samples, as it would be degraded along the water network.

Figure 6 presents the results for residual chlorine in water samples collected from five lines from November 2022 until May 2023. This shows that the residual chlorine varies between 0 and 3 ppm. Indeed, a free chlorine level of zero can signal the sudden disappearance of residual chlorine, suggesting that the water is contaminated with large amounts of organic matter and that maintaining the residuals is difficult. The absence of chlorine in the distribution system indicates that the water requires high oxidation for the treatment of bacterial regrowth or the formation of biofilms. This could happen due to pipe breakage in the network and the mixing of drinking water with sewage. Figure 6 also shows that the highest residual chlorine concentration of 3 ppm occurred in December 2022 for the first line, compared to the lowest residual chlorine concentration across the different lines during the tested period. The observation of zero free chlorine levels at some sampling points is primarily attributed to increased demand for chlorine. In this regard, Fiello et al. (2014) reported that rapid disinfection depletion occurs when the reaction between chlorine and organic precursors exceeds the initial dose, leading to a complete loss of residual protection and increasing the risk of microbial spread in the distribution network.

The detected reduction of residual chlorine concentrations from winter to summer across the selected water distribution lines can be ascribed to the combined effects of water temperature increases, pH variations, higher turbidity levels, and fluctuations in electrical conductivity, which cooperatively influence the efficiency of chlorine as a disinfectant in water systems. First of all, the water temperature considerably impacts chlorine stability and residual concentration. Higher water temperatures can accelerate chlorine decay. Accordingly, the residual chlorine levels tend to drop from winter (November to January) to summer (March to May) (Fig. 6). This behavior aligns with increased water temperatures during the warmer months (Fig. 2), which can cause faster chlorine dissipation and lower chlorine levels, as shown in Fig. (6). Second, pH levels impact the form of chlorine present in water. Higher pH levels favor the formation of hypochlorite ions, which are less effective as disinfectants. Indeed, the pH presents relatively stable values around neutral levels (7-7.5) throughout the months (Fig. 3). However, the minor increase in pH detected in warmer months can contribute to decreased residual chlorine concentrations (Fig. 6). Third, turbidity can also affect chlorine's efficiency by providing surfaces for bacteria to adhere to, which can shield them from disinfection. Referring to Fig. (4), the turbidity shows a considerable increase in January 2023, coinciding with lower residual chlorine levels (Fig. 6). Indeed, a greater value of turbidity can consume chlorine and mitigate its availability as a disinfectant. The subsequent months show lower turbidity (Fig. 4), which would correlate with higher residual chlorine concentrations during those periods. Fourth, the electrical conductivity is an indicator of the ion concentration in water, which can affect chlorine behavior and reactions. Referring to Fig. (5), the EC values present a general increasing trend from winter to summer months. Greater EC can indicate progressive ionic content, which can improve chlorine reaction rates and practically cause a faster depletion of residual chlorine. This behavior supports the observed reduction of chlorine levels during the warmer months (Fig. 6). Thus, the interplay of the studied influential factors can represent the difficulty of achieving efficient residual chlorine levels in water distribution systems. To adequately ensure perfect disinfection and water quality, continuous adjustment of these influential factors is vital, mainly as environmental conditions can change from winter to summer. Based on the obtained results for residual chlorine levels, there is a need to implement a proactive water management strategy to mitigate the effects of these influential factors on residual chlorine concentrations.

Figures (7-12) show the intensity of residual chlorine concentrations along the five selected water distribution lines from November 2022 to May 2023 using a GIS program. These figures provide visual representations of chlorine levels at different points along the pipeline network, which can highlight clear fluctuations in water quality. Figure (7) depicts the chlorine concentration across the network in November 2022. The maximum registered residual chlorine concentration is 3.29 ppm, signifying efficient chlorination in certain areas. The distinguished residual chlorine concentrations can be observed around Al-Rahma and Al-Huwaider regions, indicating effective chlorine retention in these areas. However, lower residual chlorine concentrations can be observed near the end of the distribution line at the endpoint of Al-Thahir.

The field-measured data demonstrated in Table 3 can summarise monthly mean residual chlorine concentrations (ppm) for five selected distribution lines that served as the key input for the spatial analysis. Using ArcGIS Pro, these values were combined into a geospatial framework to develop spatial distribution maps, presenting the area chlorine decline patterns.

Referring to the results shown in Fig. (8) for December 2022, the chlorine intensity shows slight variations compared to November 2022. The greatest residual chlorine concentration is 2.48 ppm, compared to 3.29 ppm in November 2022. More importantly, the distribution of residual chlorine remains comparatively uniform, with moderate levels observed in areas such as Al-Rahma, compared to other areas with reduced residual chlorine intensity, such as Al-Wathba. The lowest residual chlorine of 2.15 ppm is observed near the end of the lines, which may indicate potential issues with chlorine retention or water stagnation.

Considering the data presented in Fig. (9) for January 2023, a slight increase in residual chlorine of around 3.1 ppm is observed. In this regard, Al-Huwaider demonstrates an improved residual chlorine level compared to the end of distribution lines, which still have reduced residual chlorine levels of 0.0028 ppm. Accordingly, this would ascertain the fact of a number of remaining challenges that affect the consistency of distribution.

Fig. (10) of March 2023 signifies a peak in residual chlorine concentration, where the maximum recorded concentration is 3.99 ppm (efficient chlorination) in Al-Huwaider compared to the ends of the network that exhibit diminished residual chlorine levels, with values as low as 0.0020 ppm.

In April 2023, Fig. (11) shows a considerable decrease in residual chlorine. In this aspect, the maximum recorded concentration drops to 1.67 ppm, signifying potential challenges  in  preserving  chlorine  levels.  Specifically, Fig. (11) depicts that some areas have moderateresidual chlorine levels and others are approaching critical levels. This reduction of residual chlorine can indicate a potential issue in water treatment or distribution effectiveness.

Finally, Fig. (12) for May 2023 shows a continued trend of reduced residual chlorine concentrations, with a maximum of 2.67 ppm, an improvement from April 2023 but still lower than earlier months. More importantly, the ends of lines exhibit the lowest values (1.01 ppm), signifying persistent challenges in chlorine delivery to these areas.

Figure (13) shows the relationship between the distance traveled by water and the residual chlorine for five selected lines during the testing period from November 2022 to May 2023. It can be stated that the higher the distance traveled (from 1500 m to 3000 m), the lower the concentration. In other words, the percentage of free chlorine decreases with distance from the water source (the place of chlorine treatment). The reduction of free chlorine at 2000 m can be attributed to the possibility of its evaporation due to the long distance. To systematically resolve this issue and ensure that sufficient quantities of water reach distant points with high residual chlorine levels, the water quality supervisors constructed booster stations at a distance of 1500 m from the five lines to strengthen water pumping to distant areas, in addition to increasing the free-chlorine dose. This can be attributed to the increase in free chlorine concentrations in water samples at 3000 m, as can be seen in Fig. (13). Referring to the above discussion, it is essential to clarify that the high residual chlorine values (ranging between 3 and 5 ppm) observed at certain sampling points represent locations in the immediate vicinity of the main pumping station. These increased initial chlorine concentrations are intentionally preserved by the plant operators to compensate for the significant decay during distribution and guarantee that an adequate residual level (0.2-0.5 ppm) reaches the ends of the network. In other words, the observed field values reflect the natural gradient from the source to the terminals.

3.6. Bacteriological Tests

When the Plate count exceeds the permissible limit (100 according to the Iraqi Standard Specification for Drinking Water), it suggests bacterial contamination in the water. This can be an indicator of various possible issues, including contamination of the water network. Referring to the data in Fig. (14), it is evident that bacterial contamination (Plate count) exceeds the permissible limits of the Iraqi Standard Specification for Drinking Water. In this regard, contamination was recorded in several lines within the study area, especially in the fourth line, which spans the area between the water project and Al-Aqsa Mosque in Al-Rahma district. This area is characterized by inadequate services and frequent tampering by residents with the water network, leading to occasional pipe breaks and, consequently, contamination of drinking water with sewage and other pollutants. More importantly, the presence of bacterial contamination across multiple areas can indicate insufficient residual chlorine to kill bacteria. Water leakage from external sources, such as sewage or other environmental contamination, can also lead to contamination of the water network. It is crucial to address and resolve these issues to ensure safe, clean drinking water is provided to the affected areas. Proper maintenance and repair of the water network, as well as raising awareness among residents about the importance of preserving water quality, can help mitigate bacterial contamination and ensure the safety of the drinking water supply.

Clearly, while total coliforms and high plate counts were detected in low-chlorine areas, E. coli was absent. This suggests that the presence of the bacteria can be ascribed to environmental intrusion or biofilm regrowth in the old pipe network rather than direct recent contamination. From a public health perspective, this indicates a decline in water quality during distribution, requiring a more vigorous secondary disinfection strategy.

Figuers (15 and 16) show the MPN Coli/100 mL and total MPN/100 mL for the five lines in water samples. Although Fig. (15) indicates the absence of E. coli, a total MPN/100 mL was detected in Fig. (16), which should not be detectable in any 100 mL water sample.

3.7. Statistical Correlation Analysis

Preliminary statistical evaluation using Pearson correlation analysis (Table 4) ascertains a vigorous relationship between the examined environmental factors and residual chlorine concentrations. A considerable negative correlation was indicated between chlorine concentration and transmission distance (r = -0.842, p < 0.001). This indicates that the disinfectant is continuously consumed as the water moves through the Baquba central water treatment plant. Moderately negative correlations were also detected for water temperature (r = -0.450, p = 0.012) and turbidity (r = -0.421, p = 0.021). These results indicate that thermal kinetics and suspended particles also meaningfully contribute to chlorine degradation, while distance is the most important factor. However, pH and electrical conductivity showed no statistically significant effects, suggesting that they are secondary factors within the observed limits of this research.

To further quantify the relative contribution of each significant factor, a multiple linear regression (MLR) model was developed, as shown in Table 5. The model showed high predictive power (R2 of 0.78, p<0.001), explaining 78% of the variation in residual chlorine reduction. Quantitative identification confirms that the 'water age' associated with long-distance transfer is the key driver of chlorine loss in the water network. It mathematically indicates the regression constant (4.852) for the initial dose, compared with the negative coefficients for distance, temperature, and turbidity. Indeed, this would indicate the rate of chlorine decay as the water moves towards the edge of the network.

Referring to the above statistical correlation analysis, the detected spatial and temporal variations in residual chlorine concentrations in the water distribution network in Baquba are not just isolated trends, but are deeply rooted in the following local hydrology and operating environment:

(1) The source water of the Diyala River depicts notable variations in turbidity, particularly throughout the rainy season. The high-turbidity level delivers a protective shield for microorganisms and consumes chlorine through adsorption on suspended substances. Even though this necessitates a higher initial dose, these particles hasten the chlorine reduction as the water moves through the network.

(2) The robust negative correlation between water chlorine levels and water temperature signifies the climatic features of the studied area. Elevated temperatures catalyze the chemical oxidation of organic materials and increase disinfectant evaporation, which would explain near-zero residues at network loops (e.g., Jerf al-Malh and al-Tahrir) throughout hot periods.

(3) The majority of the Baquba pipeline network consists of old ductile and cast-iron pipes. These pipes can form internal corrosion scales and biofilms over time, continuously increasing the chlorine demand. Indeed, the contact time between water and the pipe walls increases as the transmission distance increases, which can degrade the contact before it reaches end users.

(4) The specific increases in chlorine concentration at specific points (for example, near the 3000 m mark) would indicate interference by intermediate booster stations. These measures are important to counteract cumulative reduction and certify that even remote areas preserve minimum levels of residual chlorine.