**4. Discussion**

(3)

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 BMWP [138.854 7.69 Nitrates 123.51 Nitrites 0.18 Ammonia 2.17 Total N 33.11 Total P 0.80 Sulfates 1.18 Color 0.14 Alkalinity 0.28 Chlorides 1.85 BOD 24.96 DO 0.18 Conductivity 2.19 %DO].

The BMWP values for their validation and regional extrapolation (**Figure 5**) span the whole range of water quality classes: from "Very bad, extremely polluted" (Tlatenchi) to "Excellent" (Arriba Chalchihuapan).The observed and expected BMWP values for the nine index validation sites in the Apatlaco River and the seven regional in the neighboring river subbasins (Amacuzac, Cuautla and Yautepec) were calculated using the previously BMWP scores calibrated and the derived multiple linear regression model. Four sites of the BMWP values lay outside the confidence limits (*α* = 0.05) for the BMWP linear regression models (**Figure 4b**). Pearson's correlations were calculated (*α* = 0.05) for the BMWP observed and expected values for the index validation and regional extrapolation sites (**Figures 4c** and **d**). As in the previous analysis, acceptable (*p* = 0.048) values were calculated for the index validation, whereas the regional extrapolation showed weaker correlation (*p* = 0.091). Three out sites of the nine independent sites were outliers for the calculated multiple linear

**Figure 5.** Scores of the BMWP for the validation and range extension study sites. A: Excellent; B: Good, not sensible affected; C: Regular, moderate pollution; D: Bad, polluted; E: Bad, very polluted and F: Bad, extremely polluted.

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5

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**3.4. Index validation and regional extrapolation**

regressions BMWP model (**Figures 4c** and **d**).

expected

50 Water Quality
