3.1 Environmental parameters
To represent, classify, and discuss the current situation of different Lake Mariut Basins (LMBs) and their drainage water system (drains and canal), the cluster analysis was employed using the obtained data of water quality parameters and nutrients at the monitoring locations covering the study area. The cluster analysis showed that stations I and III (center and northwest corner of MB, respectively) had their unique characteristics with the dissimilarity of 100, and 60 %, respectively, from the study area (Fig. 2). The feeding UD and NC are grouped in one cluster, while LMBs and the diverted drains are to some extent close to each other with a similarity (> 60 %). Consequently, the present results can be illustrated in terms of three categories as follow LMBs, diverted drains, and feeding UD, and NC.
LMBs were characterized by their shallowness with an average of 1 m, in addition to the diverted drains from MB (QD and WWTP effluent). While the LM drainage system UD and NC were deeper (with about 3 m depth), Fig. 3a. It worth mention that during the present study the surface water in UD was about 1 m lower than that recorded earlier in 2013 ( Shreadah et al., 2020). Reflecting shortage of the agricultural drainage water by amount reaching about 3 mcm/d to be reused after treatment for plantation of about a half-million acre at west desert (Ministry of the Water Resource and Irrigation, personal communication). This has led some parts of the lake to get dry.
The obtained results show that the water temperature of LMBs was generally varied between a minimum of 18.8°C in winter and a maximum of 30.1°C in summer. This range was coincided with those obtained by (Farouk et al. 2020), and was within the desirable optimum condition for fish culturing (Bhatnagar and Devi 2013; El Zokm et al. 2018).
Also, the water of LMBs was nearly transparent at both FB and SWB (with STD/TD of > 90%) while in MB water the STD/TD % was less, average about 50, Fig. 3b. On other hand, the drainage water of the deep UD and NC and each of the polluted shallow QD and WWTP drain were even lower 23 ± 14 %. The last was in agreement with that recorded by (Shreadah et al. 2020).
Regarding DO, extremely wide fluctuation in the concentrations over the study area happen, from nil in the effluent polluted waters of QD and WWTP beside that in the UDdownstream part and NC to 12.9 ± 4.5 mg O2/l (oversaturation, of average % oxygen saturation 169) only in FB, Fig. 3c. Therefore, the order of abundance in DO in the study lake was: FB (13.0 mg O2/l), > NWB (10.5 mg O2/l) > SWB (6.0 mg O2/l) > MB (2.6 mg O2/l). This is in agreement with that mentioned by (Abd El-Alkhoris et al. 2020). The standard guideline suitable for aquaculture purposes (Rachman and Adi, 2005) is > 3 mg/l. Therefore, MB water is in a critical stage particularly at the zones of stations I and II, where H2S gas was detected in summer. It seems likely that relatively high temperature accelerates the aerobic decomposition rate of organic matter. Also reflects the existence of remnant oxygen-consuming matter mostly those impeded with the old bottom spoiled organic-rich sediments (El-Rayis et al, 1994). The sluggishness of the water in this basin could be another reason. Further the current basin is slightly feeded with less oxygenated water at its lonely opening SW from UD. Certainly, a solution for such a critical problem is urgently needed to regain the health condition of this important basin and therefore for the fish catch.
Focusing drainage system, the adverse effect of the high organic matter load, not only restricted on DO depletion and appearance of malodorous H2S gas at each of QD and WWTP drain (average 7.4 ml H2S/l) but also responsible for converting the oxic water of the proper NC and UDdownstream into anoxic one, Fig. 3d.
By highlighting the DO content of UD water before joining MB (station IX), in the present work there was a remarkable decline to about one third (2.0 mg O2/l equivalent 25 % of O2 saturation) relative to the previously recorded in 2007 and 2013, where the respective mean values were 6.5 and 8.8 mg O2/l (equivalent to 75, and 102 % oxygen saturation (Shaaban, 2010; Shreadah et al., 2020, respectively). This drastic decrease often coincided with the latest strategy of the Egyptian Ministry of Water Resources and Irrigation in abstracting about 3 mcm/d from UD which in turn has led to lowering the water level to -325 cm (below the sea level) in this drain. This situation consequently has led part of the DO depleted water of SWB5000 to drain into the upper reach of UD. These often are the causes of the general decline of DO content noticed in the water of this essential drain.
The pH values fluctuated from 6.95, slightly acidic, in the industrial WWTP to 8.70 at FB, Fig. 3e. The change in the basins was not much and they were in the order: MB (7.48) ≤\(\)SWB5000 (7.49) < SWB2000 (7.83) < NWB (8.13) < FB (8.53). This distribution is more or less in agreement with that obtained by (Farouk et al. 2020). The relatively high values in FB reflects the role played by the presence of condensed aquatic plants there in the removal of ambient CO2 during photosynthetic activities, and thence in raising pH. This observation was previously noticed by (El-Rayis et al. 2019). It is worthy to mention, the current pH levels were within the recommended range (5.5–9.5) suggested by the Central Pollution Control Board for the inland waters (CPCB, 1995), and the proposed range (6.0–9.0) by the European Union (EU) for fisheries and aquatic life.
The lower values (7.05), noticed in QD could be due to the liberation of H2S and CO2 resulted from anaerobic decomposition processes of organic matter occurring there. It is worthy to mention that QD water always attained slightly acidic and neutral due to its anoxic sewage wastewater, which verified by an inverse significant relationship between pH and H2S (r = -0.61, p < 0.05), especially in high temperature (summer; July) coincided with increasing the decomposition (oxidation) rate of organic matter.
Seasonally, summer (July) generally exhibited the lowest pH compared with winter (November) ones. This could be a result of an increase in water temperature that accelerates the rate of decomposition of organic matter. It worthy to mention the distribution pattern pH values at waters of LMBs were previously recorded on the alkali side (Abd El-Alkhoris et al. 2020).
The salinity values of LMBs waters varied between 2.31 and 7.10 g/l and show the following decreasing order: the FB1000 > NWB3000 ≥ SWB2000 > MB6000 = SWB5000, Fig. 3f. It coincided with the previous observation by (Farouk et al. 2020), where FB (6.7 ± 0.6 g/l) is a closed basin with a high evaporation rate and no source of direct freshwater discharge and its water source is from Abis PS (with relatively high salinity about 4 g/l (Shaaban 2010). The horizontal distribution of salinity over the study area confirms the fact that the source of water of both MB (2.78 ± 0.27g/l) and SWB5000 (2.89 ± 0.06 g/l), are from UD (2.70 ± 0.04 g/l) water. Meanwhile, the waters of diverted drains, QD and WWTP, were concurrent with the freshwater salinity (1.35 ± 0.48 g/l) reflecting the role of municipal sewage water (> 88%) in decreasing the salinity of the drains water. In contrast the water of navigational canal (NC) and the agricultural drain (UD), attained high salinities.
with a relatively high residence time of LMBs The Chl a, the indicator of phytoplankton abundance and biomass (Ahmed 2020)is of values ranged between higher levels in the waters of the four basins (average 43 mg/m3) to lower levels (< 8 mg/m3) of anoxic QD, and WWTP drain. This notice agrees with that observed by Abd El-Alkhoris et al., (2020), The low Chl a content in QD and WWWTP drain reflect unfavorable conditions for algal plants due to evasion of toxic H2S. The order of Chl a abundance between LM basins is: MB > NWB > LMFB = SWB5000 and 2000, Fig. 3g. Generally, the maximum Chl a content was recorded in the summer season, coincided with low transparency and high DO concentration, reflecting the role of photosynthetic activities.
Monitoring of nutrients (N and P) levels has a significant response on the water quality and food webs at the lake (Somura et al. 2018). As well as the nutrients content determines the amount of phytoplankton and algal biomass, reflected in eutrophication status and related to total fish production (El Zokm et al. 2018).
In the current study (Fig. 4a), the TN content (with an average value > 1 mM) of the drainage water system (drains and canal) sustained at least 3 times higher than those recorded at different LMBs (with an average value < 0.4 mM). Moreover, most of the TN was in ON form in the study area, particularly, in the warm season (July). The order of TN abundance was: SWB5000 (7.2 mg/l, 0.51 mM) > MB (6.8 mg/l, 0.49 mM) > NWB (3.3 mg/l, 0.24 mM) ≥ FB (3.1 mg/l, 0.22 mM) > SWB2000 (2.6 mg/l, 0.19 mM), which reflect the declining in the TN content regarding to previous studies (Abd El-Alkhoris et al. 2020) by at least 25%.
The distribution pattern of DIN levels revealed that the least DIN values were at FB (0.02 mM) followed by NWB (0.07 mM), SWB (0.13 mM), and MB (the relatively high DIN content with an average of 0.19 mM). Moreover, July (hot season) attained the lowest values of DIN (< 0.1 mM) compared with relatively higher ones in November (cold season). Most anoxic waters of the diverted drains (QD and WWTP), downstream waters of UD and NC were characterized by the elevated DIN content relative to aerated stagnant waters of LMBs. On other hand, the downstream of the running water of both UD and NC were, always, exhibited an elevated DIN content than their upstream waters.
The NH4 has constituted a considerable part of DIN, particularly in the anoxic waters (> 90%) as illustrated in Fig. 4a. revealing the reducing conditions. This was an indication of the ammonification and/or deamination processes, which concurrent with a decline in the NO3− concentration.
Like TP, the current TP concentrations of LMBs were generally lower (by about three times of magnitude) compared with those of the drainage water system (Fig. 4b). Regarding LMBs, the order of abundance of TP was MB (0.92 mg/l, 0.030 mM) > FB (0.75 mg/l, 0.024 mM) > NWB = SWB5000 (0.49 mg/l, 0.016 mM) > SWB2000 (0.29 mg/l, 0.009 mM). However, MB showed a relative decrease in TP levels when compared with the previous studies (Abd El-Alkhoris et al. 2020), this situation was reversed for the other basins.
The extremely elevated levels of TP (> 0.07 mM) were observed at the diverted polluted drains QD and WWTP, beside the downstream part of UD (Fig. 4b). The high TP levels in the diverted drains are due to the massive amount of sewage discharge usually loaded with P.
The percentage of DIP to TP was higher in summer (July) relative to that in winter (November), this could be attributed to the increasing of domestic discharge that enriched with polyphosphates (in commercial soaps and detergents) which are quickly hydrolyzed and yield to the production of orthophosphate species in aqueous solution.
Comparing the present study results with the guidelines of the Egyptian Ministry of Water Resources and Irrigation “law 48/1982,” for protection of the Nile River and waterways from pollution, i.e., decree No. 49 in the amended executive regulations of the law by Minister Decision No. 92/2013 (Table 2) revealed that MB and SWB showed elevated levels of most nutrient salts (NH4+, ON, TN, and TP) than those recommended by EEAA, however average DO concentrations of MB was lower than permissible limits the pH values were within the recommended range. On other hand, the other two lake basins' waters (NWB, and FB) were within the allowable boundaries for most parameters excluding ON content.
3.2 Trophic Status
The trophic state can be used for evaluating the efficiency of the restoration program, and for lake categorization. This can be accomplished by applying three definite approaches, namely (1) trophic state criteria (TSC); (2) trophic state index (TSI); and finally, the nutrient loading criteria (Premazzi and Chiaudani, 1992).
-
Based upon the TSC (first approach), some proposed parameters were used for evaluating changes in the ecological conditions of lakes and for qualitative description of the trophic status. The TSC includes abiotic; STD, DO, TP, TN, organic- and inorganic-N; in addition to biotic parameters as Chl-a, (Premazzi and Chiaudani 1992; Thomas et al. 1996; Wetzel 2001b).
The trophic statuses of the studied lake waters relative to standards were presented in Fig. 5. It reveals that although waters of both NWB and FB were supersaturated with DO and contained relatively low concentrations of inorganic-N forms, their waters were identified as hyper-eutrophic relative to other standard parameters. While MB water was categorized as hyper-eutrophic regarding suggested parameters. Additionally, the diverted drain and effluent of QD and WWTP were recognized as dystrophic, with poly-humus, refractory dissolved organic and brown water which decrease the attenuation of light, causing the decline in Chlo-a concentration, (Carpenter and Pace 1997)
-
The calculated TSI using these formulas developed by Carlson (1977) that are based on STD, TP, and Chl-a are presented in Fig. 5. The results indicated that the TSIs, according to STD and Chlor-a, most of the study areas were grouped as eutrophic waters. On contrary, the TSIs (TP) were classified the study area as hyper-eutrophic. Moreover, the order of TSI values: TSI (TP) > TSI (Chl-a) = TSI (STD), proposed zooplankton grazing or/and N- content, are growth limiting factors of algal biomass production (Brown and Simpson, 2001).
-
The nutrient loading criteria according to the Vollenweider model (Vollenweider 1968, 1976) were applied to compare the present study TN and TP loading levels of UD at its upstream and downstream (UDus and UDds,) respectively with those corresponding permissible and dangerous loading levels (Wetzel, 2001) that required to maintain lake in a steady state as a function of its mean depth (Table 3). The results revealed that UDus (the feeding water source of MB after the diversion of QD and WWTP effluent) loads TP lower than the permissible and dangerous loading levels to MB. However, TN loading was exceeding the permissible levels it was lower the dangerous loading values. Moreover, levels of TN and TP entering Mex Bay through UDds were slightly higher than the permissible limit and lower than dangerous loading.
Table 2 A comparison between present study data of MB and its drains with those ones recommended according to Egyptian Law (92/2013).
Parameters
|
Unit
|
Egyptian Law (92/2013)
|
LMBs
|
Drainage system
(drains and canal)
|
FW
|
Agri. D
|
TWW
|
|
Diverted
|
Feeding
|
|
|
|
|
Domestic
|
Industrial
|
MB
|
NWB
|
FB
|
SWB
|
QD
|
WWTP
|
UD
|
NC
|
DO
|
mg/l
|
> 6
|
> 5
|
> 4
|
> 4
|
2.6
|
10.5
|
12.9
|
6.0
|
ND
|
ND
|
1.9
|
3.4
|
pH
|
SU
|
6.5–8.5
|
6.5–8.5
|
6.0–9.0
|
6.0–9.0
|
7.5
|
8.1
|
8.5
|
7.7
|
7.4
|
7.1
|
7.5
|
7.6
|
ON
|
mg/l
|
< 1
|
-
|
-
|
-
|
4.2
|
2.3
|
2.8
|
3.1
|
9.8
|
12.0
|
8.9
|
9.1
|
NH3
|
mg/l
|
< 0.5
|
-
|
-
|
-
|
2.0
|
0.2
|
0.3
|
1.2
|
6.3
|
7.8
|
2.4
|
3.0
|
NO3
|
mg/l
|
< 2
|
-
|
-
|
-
|
0.5
|
0.8
|
0.1
|
0.4
|
ND
|
0.3
|
0.7
|
0.6
|
TN
|
mg/l
|
< 3.5
|
15
|
-
|
-
|
6.8
|
3.3
|
3.1
|
4.9
|
16.1
|
20.1
|
12.3
|
12.8
|
TP
|
mg/l
|
< 0.5
|
3
|
-
|
-
|
0.9
|
0.5
|
0.8
|
0.4
|
2.3
|
2.1
|
0.9
|
1.0
|
FW = guideline for fresh waters subjected to industrial wastewater discharge Agri. D = guideline for agricultural drains ND= not detected |
− = not mentioned criterion TWW = Treated wastewater before discharge into non−freshwater aquatic environment |
MB= Main Basin NWB = North West Basin FB= Fishery Basin SWB = South West Basin QD = Qalaa Drain |
WWTP = West Wastewater Plant effluent UD = Umum Drain NC = Nubaria Canal |
Bold numbers represent the higher values than recommended guidelines |
Table 3 Values of the permissible loading levels for TN and TP in g/m 2 yr as well as those values of Umum Drain upstream and downstream (UD us and UDds, respectively) in the present study.
Mean depth
|
Wetzel, (2001)
|
Present study
|
Permissible loading
|
Dangerous loading
|
MB from UDus
|
Mex Bay from UDds
|
N
|
P
|
N
|
P
|
N
|
P
|
N
|
P
|
< 5 m
|
< 1.0
|
< 0.07
|
> 2.0
|
> 0.13
|
1.1
|
0.06
|
-
|
10 m
|
< 1.5
|
< 0.10
|
> 3.0
|
> 0.20
|
-
|
1.6
|
0.15
|
The trophic status according to Wetzel (2001) * according to Thomas et al (1996)
MB= Main Basin NWB = North West Basin FB= Fishery Basin SWB = South West Basin QD = Qalaa Drain WWTP = West Wastewater Plant effluent
UD = Umum Drain NC = Nubaria Cana
TSI = trophic state index (Wetzel, 2001) TSI (STD) = 60 − 14.41 ln (STD) TSI (CHL) = 9.81 ln (Chl−a) + 30.6 TSI (TP) = 14.42 ln (TP) + 4.15
3.3 Principal component analysis (PCA)
The principal component analysis (PCA), the multivariate method, has been applied on 16 variables (measured parameters) and 15 sampling stations. The number of factors extracted from the variables was measured regarding Kaiser's rules. This criterion retains only factors with eigenvalues that exceed one. Based on eigenvalues and varimax rotation five factors explained 86.3 % of total variability explaining 24.58%, 21.29%, 16.91%, 12.32%, and 10.23% for PCF1 (pH factor), PCF2 (NH4 factor), PCF3 (NO2 factor), PCF4 (TD factor), and PCF5 (Chl a factor), respectively, (Table 4). The loading of the variables on the five PC showed that pH, salinity, DO, and DIP are the dominant variables on the PCF1 (-0.90, -0.84, -0.80, and 0.73, respectively). The N-forms were the core variables controlling 2nd and 3rd components, where NH4 (0.94), DIN (0.93), and TN (0.68) on PCF2 and NO2 (-0.91) and NO3 (-0.74), on the PCF3. Finally, the TD (0.87) and percentage of STD to TD (-0.84), in addition to STD (0.75) and Chl a (-0.87) were the dominant variables on PCF4 and PCF5, respectively.
However, most dominant variables of PCF1 were negatively correlated with components and positively correlated to each other. Likely, high significant linear relationship between DO and pH at the meantime the inverse relationship between H2S with both DO and pH, indicated the presence of liberated H2S and CO2 weak acids resulted from an occurrence of oxidation (decomposition) processes of organic matter and exhaustion of DO (El-Rayis et al. 2019).
Table 4 Varimax rotated component matrix of the studied parameters in Lake Mariut Basins and their drainage system
Rotated Component Matrix a 2018–2019
|
|
Component (r = 0.46, p > 0.01)
|
1
|
2
|
3
|
4
|
5
|
TD
|
-0.01
|
0.04
|
-0.22
|
0.87
|
0.10
|
STD
|
-0.31
|
-0.32
|
-0.21
|
-0.15
|
0.75
|
STD/TD
|
-0.17
|
-0.22
|
-0.02
|
-0.84
|
0.37
|
Water temperature
|
0.27
|
-0.52
|
0.57
|
0.41
|
0.09
|
pH
|
-0.90
|
-0.24
|
0.02
|
-0.16
|
0.12
|
Salinity
|
-0.84
|
-0.30
|
0.11
|
-0.10
|
0.21
|
DO
|
-0.80
|
-0.39
|
0.20
|
0.03
|
0.00
|
H2S
|
0.67
|
-0.11
|
0.55
|
-0.09
|
0.07
|
Chlor-a
|
-0.06
|
-0.29
|
0.05
|
-0.02
|
-0.87
|
NH4
|
0.28
|
0.94
|
0.16
|
0.04
|
-0.02
|
NO2
|
0.12
|
0.03
|
-0.91
|
0.10
|
0.12
|
NO3
|
0.04
|
-0.24
|
-0.74
|
0.33
|
0.23
|
DIN
|
0.30
|
0.93
|
-0.03
|
0.10
|
0.03
|
TN
|
0.36
|
0.68
|
0.00
|
0.49
|
0.12
|
DIP
|
0.73
|
0.15
|
0.50
|
0.20
|
0.07
|
TP
|
0.51
|
0.54
|
0.54
|
0.10
|
0.01
|
Variance %
|
24.58
|
21.29
|
16.91
|
13.32
|
10.23
|
Cumulative %
|
24.58
|
45.86
|
62.78
|
76.10
|
86.32
|
Extraction Method: Principal Component Analysis.
Rotation Method: Varimax with Kaiser Normalization.
|
a. Rotation converged in 18 iterations.
|
3.4 Comparative evaluation of water quality of MB
To evaluate the efficiency of wastewater effluents diversion process on the water quality of MB with time (as MB-rehabilitation step), it can be employed by comparing the current state (present study, 2018) with the older ones in 2013 (Shaaban 2015; Shreadah et al. 2020), with five years interval. The average concentration values of selected parameters during 2013 and 2018 are presented in Table (5). However, the observable decreasing trend in the concentrations of H2S, TN, NO2, DIP, and TP by approximately 87, 44, 40, 25, and 20%, respectively, could be an improvement indicator. Those were combined with decreasing in DO concentration (by 84%), and a moderate decline in TD, transparency, and pH by 19, 11, and 2 %, respectively, and a slight increase in water salinity by 2%.
Moreover, the previous notice was statistically confirmed by applying the paired t-test using the data of the environmental and nutrients parameters between the two successive periods after three years (Shaaban 2015; Shreadah et al. 2020) and eight years (present study) of diversion. The results revealed significant temporal variations in terms of total depth, DO, H2S, and TN (p < 0.05), while the other parameters did not show significant differences, (Table 5).
Table 5 Results of the paired t-test of differences of the environmental and nutrients between the two successive periods after three and eight years of insulation of wastewater effluents away from MB.
Rotated Component Matrix a 2018–2019
|
|
Component (r = 0.46, p > 0.01)
|
1
|
2
|
3
|
4
|
5
|
TD
|
-0.01
|
0.04
|
-0.22
|
0.87
|
0.10
|
STD
|
-0.31
|
-0.32
|
-0.21
|
-0.15
|
0.75
|
STD/TD
|
-0.17
|
-0.22
|
-0.02
|
-0.84
|
0.37
|
Water temperature
|
0.27
|
-0.52
|
0.57
|
0.41
|
0.09
|
pH
|
-0.90
|
-0.24
|
0.02
|
-0.16
|
0.12
|
Salinity
|
-0.84
|
-0.30
|
0.11
|
-0.10
|
0.21
|
DO
|
-0.80
|
-0.39
|
0.20
|
0.03
|
0.00
|
H2S
|
0.67
|
-0.11
|
0.55
|
-0.09
|
0.07
|
Chlor-a
|
-0.06
|
-0.29
|
0.05
|
-0.02
|
-0.87
|
NH4
|
0.28
|
0.94
|
0.16
|
0.04
|
-0.02
|
NO2
|
0.12
|
0.03
|
-0.91
|
0.10
|
0.12
|
NO3
|
0.04
|
-0.24
|
-0.74
|
0.33
|
0.23
|
DIN
|
0.30
|
0.93
|
-0.03
|
0.10
|
0.03
|
TN
|
0.36
|
0.68
|
0.00
|
0.49
|
0.12
|
DIP
|
0.73
|
0.15
|
0.50
|
0.20
|
0.07
|
TP
|
0.51
|
0.54
|
0.54
|
0.10
|
0.01
|
Variance %
|
24.58
|
21.29
|
16.91
|
13.32
|
10.23
|
Cumulative %
|
24.58
|
45.86
|
62.78
|
76.10
|
86.32
|
Extraction Method: Principal Component Analysis.
Rotation Method: Varimax with Kaiser Normalization.
|
a. Rotation converged in 18 iterations.
|
Unfortunately, the current results reveal the subsequent improvement of MB water quality was temporary and fell short of expectations. The re-deterioration was fasted by lowering the water level of UD, the main LM-water source, with its negative impact not only on LMBs but also on the quality of the drain itself.
Accordingly, the implementation of further rehabilitation steps is highly recommended. Strengthen the east bank of UD separating it from MB is a must to keep the water surface level constant. Dredging the spoiled organic-rich sediments to lower any internal load of nutrients and deepening the basin too. As long as the wastewater of the EWTP (about half mcm/d) becomes now secondary treated it is better to allow it to flow again directly into MB. The excessive water is therefore allowed to flow over the bank into UD and not the reverse. This will replenish water loss by evaporation and/or evapotranspiration and seepage from this basin besides overcoming elevation in its water salinity.