# Integrated Assessment of Surface Water Quality in Danube River Chilia Branch
## 2 Materials and Methods
### Statistical Interpretation  
Statistical and computational analyses for nutrient concentrations from surface water of Chilia branch between 2013 and 2019 were performed using Microsoft Excel 2010 with XLSTAT 2015.1. The data included the number of observations, the minimum, maximum, median and mean values, the standard deviation, and Skewness (Pearson) and Kurtosis (Pearson) coefficients.  
Skewness index assesses the extent to which a nutrient's concentration distribution is symmetrical. If the distribution of responses for a nutrient concentration stretches toward the right or left tail of the distribution, then the distribution is referred to as skewed. Kurtosis index is a measure of whether the distribution is too peaked (a very narrow distribution with most of the responses in the center) [33].  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Quality Class**} & \multicolumn{1}{c}{**CCME WQI**} \\ \cline{2-3}  & **Values** \\ \hline Excellent & \(95\leq\) CCME WQI \(\leq 100\) \\ Good & \(80\leq\) CCME WQI \(<95\) \\ Fair & \(65\leq\) CCME WQI \(<80\) \\ Marginal & \(45\leq\) CCME WQI \(<65\) \\ Poor & \(0\leq\) CCME WQI \(<45\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Surface water quality classes according to CCME WQI values [5; 20; 22].  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Factor/Index**} & \multicolumn{1}{c}{**Explanation**} \\ \cline{2-3}  & **Brief Description** & **Formula** \\ \hline \multirow{5}{*}{\(\begin{array}{c}\text{F}_{1}\\ \text{(Scope)}\end{array}\)} & Assesses the extent of water quality guideline non-compliance over the time of interest; it is expressed by the percentage of variables (chemical indicators) that do not meet the water quality standards & \(\begin{array}{c}\text{F}_{1}=\text{(Number of failed}\\ \text{variables/Total number of}\\ \text{variables)*100}\end{array}\) \\  & \multicolumn{1}{c}{(\"failed variables”)} \\ \hline \multirow{5}{*}{\(\begin{array}{c}\text{F}_{2}\\ \text{(Frequency)}\end{array}\)} & Assesses the frequency by which the objectives are not met; it is expressed by the percentage of individual tests that do not meet the quality standards (“failed tests”) & \(\text{F}_{2}=\text{(Number of failed tests/Total}\\ \text{\begin{array}{c}\text{F}_{3}\\ \text{(Amplitude)}\end{array}\)}\) & Assesses the amount by which the objectives are not met; it is calculated by an asymptotic function that scales the normalized sum of excursions from objectives (_use_) to yield a range between 0 and 100 & \(\text{excursion}_{i}=\text{(Failed test value i/}\) \\  & excursion & \(\begin{array}{c}\text{failed test from the guideline}\\ \text{Represents the collective amount by}\\ \text{which individual tests do not reach the standards (are out of compliance)}\end{array}\) & \(\text{nse}=\text{($\begin{array}{c}\text{\sum excursion}_{i}$)/number \\ \text{of tests}\end{array}$}\) \\  & nse & \(\begin{array}{c}\text{Combines three measures of variance}\\ \text{(scope, frequency and amplitude) of}\\ \text{excursions from objectives to produce a}\\ \text{single unless number representing the overall water quality at a site relative to}\\ \text{the benchmark chosen}\end{array}\) & \(\begin{array}{c}\text{CCME WQI}=100-\left[\sqrt{\text{(F}_{1}{}^{2}+\text{F}_{2}{}^{2}}\right. \\ +\text{F}_{3}{}^{2})/1.732\right]\\ \text{CCME WQI}\end{array}\) & \(\begin{array}{c}\text{(1)}\\ \text{Values}\end{array}\) \\  & & \\ \hline \end{tabular}
\end{table}
Table 3: Detection Parameters for CCME WQI evaluation [22; 23; 26].