**3. Pollutants' source identification and risk assessment**

To determine the possible pollution sources of DDT, the approach by Kilunga et al. and Mohammed et al. [18, 40] (that classified sediments with (DDD +DDE)/ DDT ratio greater than 0.5 as historic and ratio less than 0.5 as recent input) was applied. The ratio of DDE to DDD was used in this study to give an idea of the degradation pathway of DDTs in the sediment [41]. DDE/DDD ratio greater than 1 was indicative of anaerobic degradation of DDT into DDD while the ratio less than 1 implies DDT conversion into DDE via aerobic degradation [42].

 The pollution data obtained compared with effects range-low (ER-L) and effects range-median (ER-M) values (sediment quality guidelines, SQGs) provided by MacDonald et al. and MacDonald and Ingersoll [43, 44] were used to evaluate the ecotoxicological significance/potential impacts of trace metals, PAH, PCBs, and DDT on benthic organism. ER-L represents the value at which toxicity may be observed in sensitive marine species (i.e., pollution is at a level that may not cause harm to aquatic environment) and ER-M represents the concentration below which adverse effects are expected to occur.

## **4. Results and discussion**

#### **4.1 Nutrient pollution**

 Results of this study showed high values of nutrient concentration in stations within estuaries compared to oceanic water stations (**Figure 2a–d**). The highest value of phosphate concentration was recorded in Kisite (304.82 μg/L) followed by River Sabaki (205.79 μg/L) and the least was in Kilifi (7.52 μg/L) whereas the highest values of ammonium were, however, recorded in Mombasa stations of Tudor creek (81.71 μg/L). The stations in Mombasa were also observed to have high nutrient levels compared to Malindi and Lamu (**Figure 2d** and **b**), a condition that can be attributed to the high population associated with municipal and industrial wastes in Mombasa compared to the other towns. It was observed that the nutrient levels for stations in Tana river estuary, which was hypothetically thought to be receiving higher nutrient input from the agricultural and residential population in the river catchment, were strikingly low (**Figure 2a**). This observation of low nutrient and chlorophyll-a concentration can be explained by the presence of several dams, which form the Seven Folks Dams Scheme. These dams are known to settle down suspended materials with adsorbed nutrients [45], therefore reducing the transportation of these sediments and nutrients downstream. Nutrient ranges recorded

#### **Figure 2.**

*Showing nutrient phosphates (μg/L), nitrates (μg/L), ammonia (μg/L), and chlorophyll-a (μg/L) distribution pattern observed in: (a) south coast estuaries: Umba, Ramisi, Sabaki, Mwena, and Tana; (b) north coast open water, stations: Kilifi, Malindi, and Lamu; (c) south coast shoreline area: Kibuy (Kibuyuni), Shimoni, Wasini, Sii Island, Gazi, and Kisite; and (d) peri-urban stations of Mombasa: Mtwapa, Tudor, and Makupa.* 

 in this study are similar to those realized by [46, 47] in the Indian Mangroves of 14–600 and 1.00–80 μg/L for nitrates and phosphates respectively. Chlorophyll-a showed a distribution pattern almost similar to that of nutrients with low values recorded in Kilifi (0.03 μg/L) while the highest ranges were observed in River Sabaki (8.19 μg/L).

#### **4.2 Spatial distribution of heavy metals**

The aluminum (Al) levels in the peri-urban creeks had a higher concentration range: 0.06–17483.66 μg/g) compared to estuaries (range: 719484.00– 9804284.00 μg/g) and pristine area Gazi Bay (335.00 μg/g). The metal concentration seemed normal since aluminum concentrations are not likely to be significantly affected by anthropogenic aluminum sources [47].

Cr concentration levels in estuaries were higher (range: 101.10–239.10 μg/g) compared to Gazi Bay (1.58 μg/g), Lamu port (range: 2.55–7.58 μg/g), and periurban creeks (range: 11.53–24.43 μg/g) with an exception of Kilindini harbor that had Cr concentration of 42,338.00 μg/g. The relatively high concentration of Cr in the river could have been caused by mining activities upstream whereas the high level observed in one of the stations in Kilindini could have resulted from the cement bulk storage at Kilindini harbor which is adjacent to the station [48].

 Pb concentration in estuaries was higher (range: 45.70–135.60 μg/g) compared to the peri-urban creeks (range: 2.75–40.65 μg/g). These high concentrations could be attributed to both natural and anthropogenic Pb point-source discharges, primarily from its use as a fuel additive, which has made it a pervasive and persistent pollutant worldwide [49]. The concentration reported in this study was, however, lower compared to Pattani Bay (range: 79.00–97.00 μg/g), Thailand Laptev, Russia (range: 16.00–22.00 μg/g; Nolting in press) and the levels previously reported for the Kenyan Coastal Zone (range: 0.13–0.56 μg/g; by [50].

#### *Decadal Pollution Assessment and Monitoring along the Kenya Coast DOI: http://dx.doi.org/10.5772/intechopen.82606*

 The concentration of Zn was higher in the peri-urban creeks (range: 23.36–264.30 μg/g) compared to the estuaries (range: 144.20–367.20 μg/g) and Gazi Bay (3.82 μg/g). High levels of Zn are associated to the dissolution of sacrificial zinc anodes used in marinas, ships, and on leisure boats at the Kenya ports and water sports areas; the adjacent Kibarani dumpsite is also a contributor since most types of wastes in the dumpsite find their way into Makupa creek [51].

 Mn level in estuaries had higher concentrations (range: 3809.00–6781.00 μg/g) compared to peri-urban creeks (range: 16.10–58.50 μg/g) and Gazi Bay (0.77 μg/g). The same trend was observed in Cu levels in which estuaries had the highest concentration (range: 63.30–169.60 μg/g) compared to peri-urban creeks (range: (0.51–52.16 μg/g) and Gazi Bay which was BDL. Cd levels in peri-urban creeks were higher (range: BDL-2.40 μg/g), compared to the pristine area Gazi Bay (0.03 μg/g) (**Table 1**). These values may be attributed to historical industrial activity around the estuaries, that is, gypsum mining in Tana River County; five cement manufacturing companies located in Athi River: Athi River Mining, Bamburi Portland Cement, East African Portland Cement, Savanna Cement, and Simba Cement; and small landfill along Sabaki river [52, 53].

The mean 210Po activities determined in the surface sediment from the three peri-urban creeks ranged between 20.29 and 43.44 Bq kg<sup>−</sup><sup>1</sup> dw. Mtwapa creek had the highest mean (37.56 ± 2.14 Bq kg<sup>−</sup><sup>1</sup> dw) compared to Makupa creek (30.42 ± 2.16 Bq kg<sup>−</sup><sup>1</sup> dw) and Tudor creek (28.64 ± 2.86 Bq kg<sup>−</sup><sup>1</sup> dw). These results are comparable to those of surface sediments from the Venice lagoon ecosystem in Italy in the range of 26–45 Bq kg<sup>−</sup>1 [54].

The high levels of 210Po activity in Mtwapa creek could have originated from the seasonal river (River Mto Mkuu) which flows into the creek discharging finer sediment from the mainland agriculture activities. In addition, high levels of silt and organic matter were found in Mtwapa creek and this may have resulted in high levels of 210Po activity since 210Po tends to adsorb on a finer particle of silt (<63 μm) due to its specific surface area compared to coarse particles (>63 μm) [49, 50]. The same observation was made by Aközcan and Uǧur Görgün [57] in Izmir Bay and Didim who attributed high levels of 210Po to Buyuk Menderes river.
