1 Introduction

Generally, harvested rainwater has lots of benefits in the world including providing water in remote areas for their approximately safe usage and it also supports the global water demands [1]. Water is one of the most important substances and it is the most abundant in life [2]. The water demand has been reduced by using harvested rainwater which is mostly collected from the rooftop run-off and it is increasingly being accepted as a means of drinking water for most rural communities in Nigeria whether it is being subjected to treatment or not [3, 4]. The most important issue in connection with using untreated harvested rainwater for drinking or other portable uses is the possible public health risks connected with microbial pathogens in the water [3, 5,6,7]. Harvested rainwater in Nigeria is rapidly being used for potable and non-potable purposes due to water scarcity which is a result of increased urbanization, industrialization, global warming and climate change [8,9,10]. Rapid industrialization and urbanization have drastically affected water quality as industries spew out large volumes of pollutants into the environment. The environmental pollution have affected the water quality in general and the quality of rainwater is also compromised by these pollutants. Numerous factors such as the leaching of roofing materials, exhaust discharges from automobiles, and the presence of dirt and faeces from flying birds, rodents’ droppings on roofs also affect rainwater quality. [5, 12]. In Southern Nigeria, due to activities connected to the oil and gas industry and the resultant effect on water and air pollution, the tendency towards contamination of rainwater is very rife. In spite of the prevalence in the use of harvested rainwater especially in the Southern part of Nigeria, there is no documented evidence of any standardization of its usage to ensure safety. It is therefore imperative that the safety and good usage of harvested rainwater be vigorously pursued. Keeping track of the quality of water can help in obtaining knowledge from natural environmental processes as well as assist in determining human impacts on an ecosystem and equally assist in ascertaining if the quality of water conforms to the set standard for drinking water quality [13,14,15,16,17].

All these measurement efforts help in ensuring that the target for environmental standards is achieved in water quality as well as in the rating of the index of water quality [18]. The index of water quality measures the scope, frequency, and amplitude of water exceedances and then combines the three measures into one score and this calculation produces a score between 0 and 100 [19]. These parameter values could be dangerous or harmful to human health if they are above the set standards [20, 21]. Water quality index (WQI) ratings were initially developed by Horton in 1965 [22] in the United States by choosing 10 of the most commonly used water physicochemical parameters and it has been widely used globally. Many modifications to the water quality index recently have been considered by different scientists and experts [23, 24]. The weighted arithmetic method classifies the water quality according to the degree of purity by using the most commonly measured water quality variables and it is widely used by various scientists [25,26,27,28] as shown in Table 1 below. Harvested rainwater has been contaminated by various atmospheric pollutions that harbours in the environment and air which was noted by various researchers [5, 29,30,31,32,33]. Several researchers also investigated some of the physicochemical and microbiological parameters of rainwater collected from industrial areas of Lagos State and their results showed that rainwater samples analysed, were heavily contaminated which was a result of anthropogenic activities which would be harmful for human consumption if not properly treated [8]. However, human activities such as agriculture, urbanization and the rate of increase in land transformation are alarming. Since, harvested rainwater is often harvested from roof run-offs and these roofs, as well as their coating materials, can become 'active' especially as the roof is ageing, these roofing materials can therefore contaminate the harvested rainwater through leaching. However, there is a need for investigation to ascertain the water quality of the harvested rainwater, and if it is within the limits for drinking water quality by NSDWQ, SON, WHO, NAFDAC, USEPA and ICMR [13,14,15,16,17, 34].

Table 1 Water quality ratings as per weight arithmetic water quality index method [25,26,27,28]
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2 Materials and methods

In this study, the right quality assurance and control processes were accounted for in a bid to guarantee the reliability of the results. Removal of contamination from potential sources led to the soaking of polyethylene material and glass wares in 1 M nitric acid for 48 h before thoroughly rinsing with distilled water [35].

2.1 Study area

The study area was Imo State and the sampling areas were two local government areas from the State, that is, Douglas Road, Owerri Municipal Council Area (urban) and Ezihe village, Uzii, Ideato North Local Government Area (rural). Douglas Road, Owerri Municipal Council is populated by government offices, Owerri main market and other shopping malls and commercial motor parks. Ezihe village, Uzii, Ideato North does not boast of any industries which can produce effluent-bearing acidic gases into the atmosphere except for palm oil milling plants, exhausts of vehicles and power generating sets as shown in Fig. 1 below.

Fig. 1
figure 1

Map showing sampling stations with their coordinates

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2.2 Sample collection and preparation

Harvested rainwater samples were collected weekly in triplicate from two local government areas from Imo State, that is, Douglas Road, Owerri Municipal Council Area (urban) and Ezihe village, Uzii, Ideato North Local Government Area (rural) in Imo State, Nigeria. After the first rain roofs run-off flush out, these rainwater samples were collected between April and July, 2017 randomly by installing a sterilized rainwater collector covered with a sieve of 0.45 micron in diameter in each designated sampling point/locations during rainfall out in the open weekly for three weeks to constitute triplicate. Rainwater samplers were also mounted 1–2 m high above the ground to avoid rainwater splash from contaminating the direct harvested rainwater from the sky. This was done without any shield on the sky to harvest the real rainwater sample which served as the control (Table 2).

Table 2 Sample code, sampling points, areas and sources of the harvested rainwater
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2.3 Digestion and analysis of the collected samples

The collected samples were grouped and named as HRWIAU/R-HRWIFU/R and divided into two portions as shown in Table 3 below. The collected samples were later transferred into properly labelled sample plastic containers which were tightly covered to avoid contamination. Some quantities of the samples were added 3.0 cm3 of Conc. HNO3 for preservation. These samples were kept in the refrigerator at 4 °C immediately after the determination of temperature, pH, TDS and Electrical conductivity at the collection point. Heavy metals in the collected samples were analysed using Flame-Atomic Absorption Spectrophotometer (Dell, ICE 3000 AA02134104 v1.30) as described in the standard method. The pH was determined using handheld pH meter model no: HI98107 (HANNA). The temperature was taken immediately using a simple thermometer calibrated in degree Celsius. Total dissolved solids (TDS) were determined using a Handheld TDS meter SUNSHINE (HI 9811–5). Electrical conductivity was determined using an EC meter HACH (model. no: 44600) electrode and was standardized with 0.01 M KCl solution. All other physicochemical properties were determined according to standard methods [36].

Table 3 Physicochemical properties, heavy metals and coliform level of harvested rainwater in corrugated iron, stone-coated tiles and asbestos roofs in Imo State, Nigeria
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2.4 Laboratory methods for bacteriological characteristics analysis

Escherichia coli was analysed according to standard method [37]. Eosin methylene blue agar was prepared, autoclaved and kept in a water bath at room temperature. Using peptone water (5.0 cm3), serial dilutions were made and pipetted into sterile Petri dishes, at the same time 1.0 cm3 of the harvested rainwater sample was added also to the petri dishes, 15.0 cm3 of Eosin methylene blue agar was added to each of the plates, thoroughly mixed by tilting and rotating the petri dish and allowed to solidify. The petri dishes were then incubated at 37 °C for 24 h. After incubation, the plates were later observed for E. coli determination.

2.5 Water quality index (WQI)

The overall water quality was carried out using the water quality index (WQI) [38]. This was done using weighted arithmetic method which classifies the water quality according to the degree of purity by using the most commonly measured water quality variables. This method has been widely used in evaluating water quality globally and particularly in research works [39]. The calculation of WQI was made by using the following Eq. (1) according to Tripaty and Sahu [40]:

$$WQI= \frac{\mathrm{\Sigma qn x Wn}}{\Sigma Wn}$$
(1)
$$Wn=\frac{1}{(Sn)}$$
(2)
$$qn=100\frac{(Vn-Vid)}{(Sn-Vid)}$$
(3)

where,

Wn = the weightage unit of each parameter obtained as shown in Eq. (2) according to the WHO set standard values; Sn = denotes the WHO set standard values for the nth parameter; qn represents the quality ratings obtained using Eq. (3). Vn represents the nth parameter of the given sampling station and Vid is the ideal value of the nth parameter in pure water (Vid for pH = 7 and zero (0) for all other parameters).

2.6 Laboratory methods for chemical composition of roof types

Determination of chemical composition of roof types (long span aluminium, asbestos, corrugated iron, Cameroon zinc, stone-coated tiles) was done using standard method [41]. Each of the roofing sheets was cut into smaller pieces and one gram (1.0 g) of each of the roofing sheets was weighed using an (G&G Electronic Scale: JJ2000Y) and was poured into beakers and melting crucible pot differently as they were labelled respectively. Aqua regia of Conc. HCl (37%, 30.0 cm3 and HNO3; 70%, 10.0 cm3) in the volume ratio of 3:1 were added to each of the labelled container based on the name of the roofing sheets were allowed for 4 h then further heated in a fume cupboard to boiling until the sample bumping stopped and was allowed in the fume cupboard for 24 h for all the digested samples to be properly dissolved then 50.0 cm3 of the de-ionized water were added to each of the digested sample differently, stirred and then kept to cool and was later filtered using Whatman filter paper into 100 cm3 volumetric flask repeatedly for three times to get a clear solution and was made up to the mark with de-ionized water and were analysed using Flame-AAS instrument.

2.7 Statistical analysis

Data generated in this study were analysed for mean and standard deviation (SD). Significant level (P < 0.05) was determined among the harvested rainwater samples from different roof run-offs and direct rainwater from the sky (control) by descriptive and one-way ANOVA and reported in two significant figures (SPSS statistical software version 20.0).

3 Results

These physicochemical parameters namely; pH, temperature, total dissolved solids, total suspended solids, electrical conductivity, turbidity, nitrate and heavy metals (iron, zinc, lead, chromium, aluminium) and E. coli were investigated in the harvested rainwater from roof types namely; Cameroon zinc roof, asbestos roof, stone-coated tiles roof, corrugated iron roof, long span aluminium roof and rainwater was also harvested directly from the sky (control) at Douglas Road, Owerri Municipal Council (urban) and in Ezihe village, Uzii, Ideato North Local Government Area (rural) both in Imo State and are shown in Tables 3, 4 below and results of the digested roof types metals concentrations (mg/kg) as shown in Table 5.

Table 4 Physicochemical properties, heavy metals and coliform level of harvested rainwater direct from the sky, Cameroon zinc and long span aluminium roofs in Imo State, Nigeria
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Table 5 Metals concentrations of the roof types
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4 Discussion

4.1 Physicochemical characteristics

The mean pH values of direct harvested rainwater from sky (control) in the urban (Douglas Road, Owerri Municipal Council) and rural (Ezihe village, Uzii, Ideato North L.G.A) areas of Imo State were 6.00 ± 0.30 and 6.50 ± 0.20 respectively (Table 4 and Fig. 2) and were slightly acidic. Harvested rainwater from Cameroon zinc, long span aluminium, corrugated iron, stone-coated tiles and asbestos roofs also showed slight acidity having mean pH values between 5.80 ± 0.60–6.60 ± 0.15 in the urban area and 5.70 ± 0.50–6.50 ± 0.15 in the rural area as presented in Tables 3, 4, and depicted in Fig. 2 above. Although pH has no direct impact on human health, however its close connection with other chemical constituents of water makes it have indirect impact on human health [6, 15, 42]. These values were in support with the earlier findings of Okudo et al. and Emerole et al. who reported of slight acidity (6.39–6.81 and 6.2) respectively for harvested rainwater samples in Owerri, Imo State and Enugu State, Nigeria [5, 43]. Also, pH values of 5.46–5.98 have been reported for harvested rainwater by Chukwuma et al. from Oko, a town in Anambra State, Nigeria [44]. This study was not in accord with the findings of Nnaji et al. who also reported higher values of pH (8.6) in Ikpa/Orba road harvested rainwater from road run-offs in Nsukka, Enugu State, Nigeria [3]. The values obtained for pH in harvested rainwater from Imo State were within the set standards of 6.5–8.5 for drinking water by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17] except in a few rainwater samples. Comparing the pH values ranging from 5.70 to 6.60 obtained in this study which showed slight acidity with the earlier findings by Foupouagnini et al. who reported that the rainwater quality in some urban and rural sites of Cameroon (Central Africa) gave pH values ranging from 4.92 to 6.41 in both areas [45]. These values obtained in this study were in supports with their obtained results though their values were not within the permissible limits of 6.5–8.5 by WHO and USEPA [15, 17]. This study was not in agreement with the findings of Owusu and Asante [46] who reported that the rainwater harvesting and primary uses among rural communities in Ghana gave pH values ranging from 6.5 to 8.5 which were in total conformity with the set standards by WHO and USEPA [15, 17].

Fig. 2
figure 2

pH values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean temperature (oC) values of direct harvested rainwater from the sky (control) in both areas of Imo State were between 25.00 ± 3.61 and 26.30 ± 1.53 °C respectively as shown in Table 4 and Fig. 3 above. Roof run-offs had temperature values ranging from 23.00 ± 3.00–25.30 ± 1.53 °C for Cameroon zinc roof, stone-coated tiles roof (23.70 ± 2.52–26.00 ± 2.00 °C), corrugated iron roof (24.70 ± 2.52–25.00 ± 1.00 °C), long span aluminium roof (24.70 ± 1.53–27.00 ± 1.00 °C) and asbestos roof (23.30 ± 2.52–24.00 ± 2.00 °C) in the urban and rural areas as shown in Tables 3, 4 and Fig. 3 above. However, the mean temperature values of all the roof run-offs when compared with the direct harvested rainwater from the sky, it showed that, there was significant difference (p < 0.05) in the results obtained which was seen in long span aluminium and corrugated iron roofs in the urban areas, although, the temperature levels were seen to be varying with time of collection and the changes of the temperature have a lot to do with the atmosphere and surroundings. These results obtained for temperature in this study were not in line with the earlier findings by Olowoyo who reported temperature values of 29.20 ± 0.22 °C in harvested rainwater from Warri axis of Delta State [47] but were in line with the findings of Ojo, Emerole et al. and Moses et al. who reported moderate temperature values in their study [6, 43, 48].

Fig. 3
figure 3

Temperature (oC) values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean TDS values (mg/L) of harvested rainwater direct from the sky (control) in both areas of Imo State were in the range of 3.70 ± 1.52—11.70 ± 3.51 respectively as shown in Table 4 and Fig. 4 above. Harvested rainwater from roof run-offs showed higher values 27.00 ± 3.00 mg/L for TDS in the corrugated iron roof in the urban area and other values ranging from 4.00 ± 2.00–12.70 ± 5.86 mg/L for TDS from the roofs run-offs in the rural area as shown in Tables 3, 4 and Fig. 4 above. The earlier findings by Chukwuma et al. from Oko in Anambra State gave a mean values ranging from 35.3–43.6 mg/L for TDS of harvested rainwater [44] which were higher than the present results in this study. Emerole et al. reported high values of TDS in the analysed rainwater samples [40] though they were within the WHO set standards for drinking water quality. Nanji et al. also reported higher values of TDS in their findings of harvested rainwater from road run-offs in Nsukka, Enugu State, Nigeria [3]. The TDS values of the harvested rainwater samples obtained in this study were within the drinking water set standard of 250–500 mg/L by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

Fig. 4
figure 4

TDS (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean TSS values (mg/L) of harvested rainwater direct from the sky (control) in both areas of Imo State were in the range of 6.00 ± 2.00–16.00 ± 4.00 respectively as shown in Table 4 and Fig. 5 above. Roof run-offs harvested rainwater gave mean values ranging from 5.67 ± 2.52–12.00 ± 3.00 mg/L for Cameroon zinc roof, 8.30 ± 1.00–9.67 ± 1.53 mg/L for long span aluminium roof, 14.70 ± 5.91–31.30 ± 3.51 mg/L for corrugated iron roof, 8.00 ± 2.65–9.60 ± 2.51 mg/L for stone-coated tiles roof and 11.30 ± 2.52–12.00 ± 3.61 mg/L for asbestos roof in the urban and rural area as shown in Tables 4, 3 above. When collaborating the values obtained from the control and roof run-offs, it was observed that, this indication is, a result of contribution from particulate matter which shows minimal concentrations since it did not reflect on the direct harvested rainwater from the sky. The earlier findings by Chukwuma, et al. from Oko in Anambra State gave a range of 95.32–150.32 mg/L for TSS of harvested rainwater [44] which was higher than the present results in this study. Emerole et al. reported high values of TSS in their studies on harvested rainwater carried out in Owerri, Imo State [43] though they were within the WHO and USEPA set standards. The TSS values of the harvested rainwater samples obtained in this study were within the drinking water set standard of 250–500 mg/L by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

Fig. 5
figure 5

TSS (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean electrical conductivity values (µS/cm) of direct harvested rainwater from sky (control) in the sampled locations of Imo State were 11.30 ± 2.08 and 7.30 ± 2.52 µS/cm respectively (Table 4 and Fig. 6). Roof run-offs harvested rainwater from Cameroon zinc, long span aluminium, corrugated iron and stone-coated tiles had electrical conductivity mean values ranging from 9.67 ± 1.52–20.70 ± 2.51 µS/cm in the urban area and 9.70 ± 1.53–24.30 ± 6.03 µS/cm in the rural area which were of low values when compared to the set standards as shown in Tables 4, 3 and Fig. 6 above. The low values of conductivity may be because the rain has less accumulation of dust, debris, other household waste and airborne aerosols that dissolve before deposition. Roof run-offs results when compared with the direct harvested rainwater from the sky (control) values, it was observed that, there was significant difference (p < 0.05) in the results obtained may be because of corrosiveness of the roof type, atmospheric pollution, vehicular emissions and other household waste that dissolved in the rainwater from the roof run-offs harvested rainwater in the urban and rural areas. The obtained results were in low concentrations and were in line with the earlier findings by Nanji et al. who reported low values of conductivity in rainwater harvested from road run-offs in Nsukka, Enugu State, Nigeria [3]. These results were also close to earlier findings by Okudo et al., Emerole et al., Eruola et al. and Ayenimo et al. who reported low degree of electrical conductivity in harvested rainwater samples from Emene, Iva Valley, Ugwogo Nike, Oke-Lantoro, Abeokuta, Ile-Ife and in Owerri respectively [5, 43, 49, 50]. The mean concentrations of electrical conductivity of harvested rainwater samples obtained in this study were all within the permissible limits of 1000–1200 µS/cm for drinking water by WHO, NAFDAC, USEPA and WHO [15,16,17, 51].

Fig. 6
figure 6

Electrical Conductivity (µS/cm) values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean turbidity values (NTU) of harvested rainwater direct from the sky (control) in both areas of Imo State were in the range of 2.59 ± 0.41 (rural)—3.53 ± 0.11 (urban) as shown in Table 4 and Fig. 7 above. Roof run-offs of the samples had mean values of turbidity ranging from 3.77 ± 0.16–6.53 ± 0.49 NTU in the urban area and 2.42 ± 0.28–5.69 ± 0.39 NTU in the rural area as shown in Tables 3, 4. The turbidity values of direct harvested rainwater from the sky (control) when compared with all roof run-offs harvested rainwater values, it was observed that there was a significant difference (p < 0.05) in the results obtained indicating influence on the rainwater quality. Stone-coated tiles roof and asbestos roof-harvested rainwater gave higher concentrations of turbidity as shown in Tables 3, 4and the high levels of turbidity observed in these roofs’ run-offs harvested rainwater were caused by micro-pollutants, dust particles and anthropogenic pollutions in these areas while Cameroon zinc roof, corrugated iron roof, long span aluminium roof and direct rainwater from the sky (control) gave lower concentrations of turbidity as shown in Tables 3, 4 and these lower values of turbidity were likely due to less suspended and dust particles transferred into the harvested rainwater. Earlier findings by Emerole et al. indicated the highest concentration for turbidity which had mean values of 23.92 ± 6.20 NTU in Owerri, Imo State [43] and the findings of Nanji et al. who also reported the highest concentration of turbidity (52.8 NTU for Ikenga road; 30.2 NTU for Campus gate; 46.0 NTU for Enugu road and 22.5 NTU for Ikpa/Orba road) in road run-off harvested rainwater in Nsukka, Enugu State [3]. The findings of Okudo et al., Ojo, Moses et al. who reported moderate values of turbidity in the harvested rainwater in Emene and Iva Valley, Enugu State; Akure, Ondo State and Uyo, Akwa Ibom State respectively were in support with some of the values obtained in this study [5, 6, 46]. In this study, the turbidity values for the harvested rainwater directly from the sky (control), long span aluminium, corrugated iron and Cameroon zinc roofs in both areas were all within the permissible limits while the values obtained for the asbestos roof (urban) and stone-coated tiles roof in both areas were above the permissible limits of 5 NTU (turbidity) for drinking water quality by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

Fig.7
figure 7

Turbidity (NTU) values of Harvested Rainwater in Imo State (urban and rural areas)

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The mean nitrate values (mg/L) of harvested rainwater direct from the sky (control) in the sampled locations in Imo State were in the range of 0.04 ± 0.01- 0.06 ± 0.00 respectively as shown in Table 4 and Fig. 8 above. Roof run-offs harvested rainwater from Cameroon zinc, long span aluminium, corrugated iron, asbestos and stone-coated tiles roofs had nitrate values ranging from 0.06 ± 0.01–0.44 ± 0.33 mg/L in the urban area and 0.05 ± 0.00–0.36 ± 0.27 mg/L in the rural area as shown in Tables 3, 4. The highest concentrations of nitrate (0.44 ± 0.33 mg/L in the urban and 0.36 ± 0.27 mg/L in the rural) in this study were observed in the harvested rainwater from corrugated iron roof though the obtained values were still within the set limits (WHO, NAFDAC, NSDWQ&SON, USEPA). Earlier findings by Emerole et al. reported high values of nitrate which ranged from 18.0–81.1 mg/L in harvested rainwater from Owerri in Imo State, which is likely due to the ambient condition at Owerri during the period of the sampling [43]. However, other findings pointed to Ojo, Chukwuma et al. and Moses et al. who reported low values of nitrate in harvested rainwater from Akure, Ondo State, Oko, Anambra State and Uyo, Akwa Ibom State respectively [6, 41, 48]. Nitrate mean values of the harvested rainwater samples were below and within the permissible limits of 15–50 mg/L for drinking water quality by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

Fig. 8
figure 8

Nitrate (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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It was discovered through comparism that, the physico-chemical characteristics results obtained in this study were not in agreement with the earlier findings by Foupouagnini et al. who concluded that, based on the results of TDS, major ion, and trace metal contents in their study in Cameroon (Central Africa), that rainwater is considered suitable for human consumption [45].

4.2 Bacteriological characteristics

Escherichia coli (E.coli) were not detected in any of the analysed samples obtained from the sampled locations in Imo State, Nigeria as shown in Tables 3, 4. This is likely an indication of less contamination due to wind-blown dirt, air pollution, dead leaves falling from trees, and faecal origin from flying birds, hence, it reflects the potential for the rainwater to be harbouring no pathogenic organisms that could be transferred into the roofs run-offs harvested rainwater. This result is in accordance with earlier findings by Emerole et al. who reported no detection of E. coli in the harvested rainwater from Owerri, Imo State [43] and that of Ojo who also reported that E. coli was not found in the rainwater samples analysed in Akure, Ondo State[6] but not in line with the findings of Nanji et al. who detected E. coli in the harvested rainwater from Ikenga Road (2 cfu/mL), Enugu road (1 cfu/mL) and Ikpa/Orba road (6 cfu/mL) in Nsukka, Enugu State in road run-off harvested rainwater [3]. Achadu et al. also reported detection of E. coli in analysed harvested rainwater samples in Wukari, Tabara State [11]. The harvested rainwater samples analysed for Escherichia coli in this study were all within the set standards of 0 cfu/mL for drinking water quality by WHO, NSDWQ and SON since there was no detection [13,14,15].

4.3 Heavy metals concentrations

The results obtained in the present study showed that, the heavy metals mean values of direct harvested rainwater from the sky (control) indicate the absence of Fe, Zn and Al in both areas and Cr in the rural area only. There were significant levels (mg/L) of Pb (0.09 ± 0.11, rural and 0.02 ± 0.01, urban) and Cr (0.79 ± 0.13, urban) as shown in Table 4 and Figs. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13 above. This significant difference could be attributed to originating from atmospheric pollution and increased air pollution from vehicular emissions at the sampling duration/location which had effects on the direct harvested rainwater from the sky (control) in the study areas which showed high levels of Pb and Cr and these values obtained were above the set standards by NSDWQ, SON, WHO, NAFDAC and USEPA for drinking water quality [13,14,15,16,17]. Heavy metals contamination in all the roof run-offs harvested rainwater showed mean values ranging from 0.00–0.50 mg/L for Fe in the urban area and 0.00–0.16 mg/L in the rural area. Zn showed values ranging from 0.00–0.06 mg/L (urban) and 0.00–0.04 mg/L (rural) and Al had values ranging from 0.00–0.63 mg/L in the urban area and 0.00–0.62 mg/L in the rural areas in the roofs run-off harvested rainwater. Pb and Cr were not detected in harvested rainwater from Cameroon zinc, long span aluminium, corrugated iron and stone-coated tiles roofs in both areas as shown in Tables 4, 3. Fe, Zn, Cr (rural) and Al showed no detection in asbestos roof rainwater in the urban and rural areas. However, long span aluminium roof, corrugated iron roof, asbestos roof and Cameroon zinc roof had the highest concentration of heavy metals in the roofs' run-offs harvested rainwater in both areas (Tables 3, 4). This could likely be attributed to the duration of sampling and the nature of the roof type having an effect on the rainwater quality (Table 5) because when rain fell, samples were collected after 5 min, to allow some debris to be washed off from the rooftops before harvesting. Asbestos roof and direct rainwater from the sky (control) gave highest concentrations of 0.72 ± 0.11 mg/L and 0.79 ± 0.13 mg/L for chromium respectively in the analysed harvested rainwater in the urban area as indicated in Tables 3, 4; this was attributed to vehicular emissions, construction of a heavy road and activities of the mechanics along the road in the sampled area. The result obtained in this study was in support of the earlier findings by Nanji et al. who reported high concentrations of chromium in the road run-offs harvested rainwater in Nsukka, Enugu State, Nigeria [3]. The mean concentration of 0.07 ± 0.05 mg/L (rural) for Pb in rainwater samples from asbestos roof and 0.50 ± 0.62 mg/L (urban) and 0.16 ± 0.17 mg/L (rural) for Fe in Cameroon zinc roof harvested rainwater analysed and 0.21 ± 0.13 mg/L for corrugated iron roof rainwater (urban area) were observed in roofs run-offs harvested rainwater which were of higher values and were above the permissible limits by NSDWQ, SON, WHO, NAFDAC and USEPA for drinking water quality [13,14,15,16,17] and are shown by comparism in Figs. 9, 10, 11, 12 and 13 above. The highest values observed in the analysed samples were due to the acidic nature of rainfalls from these areas, leaching of metals from the roofing material occurs, thereby increasing the metal concentrations on the roof run-offs harvested rainwater. These results were in supports with the earlier findings by Emerole et al. who reported higher values of heavy metals in direct rainwater and roof run-offs harvested rainwater (mg/L) of Fe (2.12 ± 1.17), Al (1.70 ± 1.83) and Pb (0.44 ± 0.36) from Owerri, Imo State, Nigeria [44]. Okudo et al. reported high values of Pb (Emene:0.58 ± 0.11and Iva Valley:0.48 ± 0.04) and Cr (Emene: 0.10 ± 0.02) in analysed rainwater samples from Enugu State, Nigeria [5] and Ayenimo et al. also reported higher values of 0.59 ± 0.29, 0.82 ± 0.14 and 1.04 ± 0.27 for Fe, Zn and Pb respectively [48] which were above the set standards of 0.30 mg/L, 0.01 mg/L, 0.05 mg/L and 0.1–0.2 mg/L for Fe, Pb, Cr and Al respectively for drinking water by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

Fig. 9
figure 9

Pb (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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Fig. 10
figure 10

Cr (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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Fig. 11
figure 11

Fe (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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Fig. 12
figure 12

Zn (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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Fig. 13
figure 13

Al (mg/L) values of Harvested Rainwater in Imo State (urban and rural areas)

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Leaching/corrosiveness capacity of the roofing metals to dissolved metals into the harvested rainwater quality when compared with the digested roofs results (Table 5) were as follows: Al, Zn, Fe and Cr (metal roofing > stone-coated tiles roof > asbestos roof) and Cr and Pb can be transported in the order of: asbestos roof > stone-coated tiles roof and particulate metals were as follows; Cr and Pb were high in asbestos roof than other roofing sheet in both areas. Due to the acidic nature of rainfalls from these areas, corrosiveness/leaching of metals from the roofing material occurs, thereby increasing the metal concentrations on the roofs run-offs harvested rainwater from Douglas road, Owerri Municipal Council (urban) and Ezihe village, Uzii, Ideato North L.G.A (rural) which gave high mean concentrations for Zn, Fe and Al which were found to be above the permissible limits for drinking water quality by NSDWQ, SON, WHO, NAFDAC and USEPA [13,14,15,16,17].

4.4 Water quality index (WQI) ratings

The harvested rainwater values obtained in this study from the urban and rural areas of Imo State, Nigeria when compared with the water quality index (WQI) ratings standards by USEPA, WHO and ICMR [15, 17, 34] which states that, the water quality within the range 0–25 are classified as excellent water quality, 26–50 as good water quality, 51–75 as poor water quality, 76–100 as very poor water quality and water quality within the range > 100 as unfit for drinking. It was seen that stone-coated tiles roof harvested rainwater were rated excellent water quality in both areas. Long span aluminium and corrugated iron roofs harvested rainwater were rated good water quality in both areas while Cameroon zinc roof harvested rainwater was rated poor water quality in the urban area and good water quality in the rural area. Direct harvested rainwater from the sky (control) was rated as unfit for drinking water in the urban area and very poor water quality in the rural area whereas asbestos roof was rated as unfit for drinking water quality in the urban area and excellent water quality in the rural area as shown in Table 6 above.

Table 6 Water quality indices (WQI) of harvested rainwater from different roof types and directly from the sky (control)
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5 Conclusions

As regards the findings of bacteriological characteristics in this study, there was no detection of E. coli in any of the analysed harvested rainwater samples in both areas and the physicochemical characteristics viz. pH, TDS, EC, TSS and turbidity were all within the set standards for drinking water quality with the exception of a few which were above the set standards. Heavy metals analyses for Pb, Cr, Fe, Zn and Al gave mean values which were below, within and above the permissible limits by WHO, USEPA, NAFDAC and NSDWQ/SON. Stone-coated tiles roof harvested rainwater in both areas and asbestos roof rainwater (rural) gave excellent water quality by the WQI ratings and unfit for drinking water quality in direct rainwater from the sky (control) in the urban area, this was a clear indication that the prevailing industrial activities in the different study areas had an effect on the rainwater quality as shown by the calculated water quality indices results. Therefore, it was concluded that, harvested rainwater from the sky directly and from different conventional roofs (long span aluminium, Cameroon zinc, stone-coated tiles, asbestos and corrugated iron) run-offs were affected by the pollutants from the sources of particulate matter due to natural and anthropogenic factors or from the manufacturing/corrosive products of the roof type. Hence, this study revealed that harvested rainwater from the sky directly or from the roof run-offs may not be suitable for direct drinking without proper treatment but could be used for other domestic, recreational, agricultural and industrial purposes as indicated by water quality index ratings. It is pertinent that further study on the measures of providing low-cost treatment plans for companies and people in the rural communities in Imo State is suggested and in Nigeria as a whole.

6 Recommendations

Based on the results of the study, the following suggestions are recommended;

  1. (i)

    It is recommended that there should be a set standard by USEPA, WHO and NAFDAC for potable and good rainwater quality to ensure safety since there is no standardization for it.

  2. (ii)

    The federal, state and local governments should launch an enlightenment campaign on the importance of rainwater harvesting as a means of water conservation and should also prioritize creating enlightenment for people on how to boil, cool and filter harvested rainwater before being used for potable usage to avoid water-borne diseases.

  3. (iii)

    Harvesting of rainwater from metallic roofs should be done with caution because when acid rain falls depending on the industrial activity of an area especially where gases are flared into the atmosphere from the refinery; it can cause corrosion and leaching of the metals from the roofs into the harvested rainwater.

  4. (iv)

    It is recommended that the use of metal roofing sheets in the building design should be in preference in Nigeria over other roofing materials since metal roofs get hot in the tropical region or season to sterilize themselves and the roof should be replaced with a new one when it is too old.

  5. (v)

    Proper environmental monitoring/measures should be adopted in the local government areas to control atmospheric pollution and anthropogenic emissions in these areas.