Index name

Said’s Index (SI)

Scientist who Developed Index: Name, Institute; Year; First Reference;

Ahmed Said1, David K. Stevens2, Gerald Sehlke3
1Department of Civil and Environmental Engineering University of South Florida Tampa, Florida 33620-5350, USA
2Civil and Environmental Department Utah State University Logan, Utah 84322-4110, USA
3Integrated Environmental Analysis Department Idaho National Engineering & Environmental Laboratory Idaho Falls, Idaho 83415-2213, USA
2004
Said, A., Stevens, D. K., & Sehlke, G. (2004). An innovative index for evaluating water quality in streams. Environmental Management, 34(3), 406–414.

Abstract (Summary):

A water quality index (Said’s Index) expressed as a single number is developed to describe overall water quality conditions using multiple water quality variables. The index consists of water quality variables: dissolved oxygen, specific conductivity, turbidity, total phosphorus, and fecal coliform. The objectives of this study were to describe the preexisting indices and to define a new water quality index that has advantages over these indices. The new index was applied to the Big Lost River Watershed in Idaho, and the results gave a quantitative picture for the water quality situation. If the new water quality index for the impaired water is less than a certain number, remediation—likely in the form of total maximum daily loads or changing the management practices—may be needed. The index can be used to assess water quality for general beneficial uses. Nevertheless, the index cannot be used in making regulatory decisions, indicate water quality for specific beneficial uses, or indicate contamination from trace metals, organic contaminants, and toxic substances.

Keywords: Big Lost River; Idaho; 303(d) list; Total maximum daily load; National Sanitation Foundation; Water quality index

Introduction

Said’s Index (SI)

The Said’s Index WQI equation was developed in two steps. The first was ranking water quality variables according to their significance. The variables included in the new WQI are DO, total phosphates, fecal coliform, turbidity, and specific conductivity. Second, several forms were tested to give DO the highest weight followed by fecal coliform and total phosphorus. The percent saturation reflects the temperature effect. Turbidity and specific conductance were given the least influence. A final form was selected that keeps the index in a simple equation and a reasonable numerical range. The logarithm was used to give small numbers that are easily used by the management decision-makers, the stakeholders, and general public as well. A sensitivity analysis was performed to test the performance and to verify that appropriate influences were given for the water quality variables as shown in Figure 1. In the final form, the powers of the variables were chosen for the WQI based on the effect of each variable on water conditions. For example, higher values of fecal coliform and total phosphorus will be very harmful for health and aquatic life. The forms of the fecal coliform and total phosphorus in the index formula were chosen to give strong responses to these effects. On the other hand, turbidity and specific conductance have linear effects, which are less sensitive for changing the values of the variables, in the index formula. This is because, for example, turbidity would not be very dangerous unless it is associated with a higher level of disease-causing microorganisms that will make fecal coliform higher as well in the formula. To calculate this index, there is no need to standardize the variables. The calculations are further simplified through the elimination of sub-indices (percent of ideal situation of each variable).

Uses and Limitation:

The index can be used to evaluate the input water to a water treatment plant because it includes the most important variables for drinking water treatment operations.

It cannot always show the impact of random short-term changes, such as a spill, except if it occurs repeatedly or for a long time. The best results with this index can be obtained in natural conditions and natural measurement sites (not downstream of river outfall). The index can be used to assess water quality for general uses. However, it cannot be used in making regulatory decisions or to indicate water quality for specific uses. The localized changes in water quality may not be immediately reflected. Another change not necessarily reflected in the index is the stream habitat. In addition, the index cannot be used to indicate contamination from trace metals, organic contaminants, or other toxic substances. The factors of water levels or stream velocities may be incorporated into a physical/chemical/biological index in the future.

Categorization Table

The index was designed to range from 0 to 3. The maximum or ideal value of this index is 3. In very good waters the value of this index will be 3. From 3 to 2, the water is acceptable, and less than 2 is marginal and remediation, if one or two variables have deteriorated, the value of this index will be less than 2. If most of the variables have deteriorated, the index is less than 1, which means that water quality is poor.

Standards Required

Any national or international water quality standards could be used in this method.

Variables Selection

The variables included in the new WQI are DO, total phosphates, fecal coliform, turbidity, and specific conductivity.

Calculation of Said’s Index :

The purosed index calculation is as follows:

where

DO is the Dissolved Oxygen (% oxygen saturation);

TURB is the Turbidity (Nephelometric turbidity units [NTU]);

TP is the total phosphates (mg/L);

FC is the fecal coliform bacteria (counts/100 mL);

SC is the specific conductivity in (MS/cm at 25 _C);

The index was designed to range from 0 to 3. The maximum or ideal value of this index is 3.

In very good waters the value of this index will be 3. From 3 to 2, the water is acceptable, and less than 2 is marginal and remediation, if one or two variables have deteriorated, the value of this index will be less than 2.

If most of the variables have deteriorated, the index is less than 1, which means that water quality is poor.

Case Studies based on Said’s Index

The Big Lost River Watershed, Idaho, is considered a relatively clean watershed. Table 4 shows some stream water quality variables and the calculated values using the proposed index for the Big Lost River Watershed.

The calculations show that the values of the index are generally more than 2 except for near Arco, which means that the water is otherwise acceptable. These streams are not mentioned in the U.S. EPA 303(d) list for impaired water bodies in the Big Lost River Watershed. However, they may not show up on the 303(d) list because “Listing Policies” differ from water quality criteria–based decision-making. These data along with data from Ott (1988) and field measurements and samples collected by Idaho Department of Environmental Quality (IDEQ) in July 2000 for the tributaries of the Big Lost River were used to depict the index for different sub-basins in the Big Lost River Watershed.

References

Said, A., Stevens, D. K., & Sehlke, G. (2004). An innovative index for evaluating water quality in streams. Environmental Management, 34(3), 406–414.