PhreeqcUsers Discussion Forum

Applications and Case Studies => Irrigation studies => Topic started by: Charlie on March 09, 2014, 01:19:15 PM

Title: Assessments of irrigation water quality
Post by: Charlie on March 09, 2014, 01:19:15 PM
Has anyone used PHREEQC for assessing water quality in terms of irrigation purposes?  Are there any recommended methods they would suggest, or special factors to consider? Thanks.
Title: Case study of irrigation water quality assessment using PHREEQC
Post by: Charlie on March 14, 2014, 12:19:36 AM
I have been reading a paper by Wanda et al, (2013). (Physics and Chemistry of the Earth 66 (2013) 51–59).  This gives a helpful and explanatory example of an assessment of irrigation water using PHREEQC.

The purpose of this study was to evaluate and ascertain the suitability of the groundwater for irrigation purposes. A number of researchers have proposed different methods of analyzing irrigation water quality data. These metrics are regarded as the most effective ways to communicate irrigation water quality.  They include:

#Total hardness (TH) calculated using mol/l.  Be aware that conversion of non-specific species data (lab data) to ppm is not compound specific (assumes only CaCO3 would ppt)

#TDS content of the water is considered satisfactory when it contains <1000 mg/l, fair if it contains between 1000 and 2000 mg/l, and inferior when >2000 mg/l.

#RSC. The excess sum of carbonate and bicarbonate in groundwater over the sum of calcium and magnesium also influences the unsuitability for irrigation. RSC value <1.25 meq/l is good for irrigation, a value between 1.25 and 2.5 meq/l is of doubtful quality and a value >2.5 meq/l is unsuitable for irrigation. Land irrigated with such water becomes infertile owing to deposition of sodium carbonate and will affect the crop yields. 

#Permeability index (PI).  The soil permeability is affected by the long-term use of irrigated water rich in Na+, Ca2+, Mg2+, and HCO3- and the soil type. Doneen (1964) classified irrigation waters in three PI classes. Class-I and Class-II water types are suitable for irrigation with 75% or more of maximum permeability, while Class-III types of water, with 25% of maximum permeability, are unsuitable for irrigation.

#Sodium concentration is important in classifying irrigation water because sodium reacts with soil to reduce its permeability. Excellent to good irrigation water (% Na+ < 60%) will not cause soil aggregates to disperse and subsequently cause reduction in permeability and are suitable for irrigation.

#The Kelly’s ratio of unity or less than one is indicative of good quality of water for irrigation.  Values greater than one suggests unsuitability of water for agricultural purpose due to alkali hazards.

#Electrical conductivity is a good measure of salinity hazard to crops, as it reflects the TDS in groundwater. High salt content in irrigation water causes an increase in soil solution osmotic pressure.  The osmotic pressure is proportional to the salt content or salinity hazard. The salts, affecting the growth of plants directly, also affect the soil structure, permeability and aeration, indirectly affecting plant growth. Electrical conductivity is considered the most influential water quality guideline on crop productivity. At higher EC, less water is available to plants.  Water are classified as excellent (C1, EC range = 100–250 μS/cm), good (C2, EC range = 250–750 μS/ cm) and doubtful (C3, EC = 750–2,250 μS/cm).

#Sodium adsorption ratio (SAR). SAR values <10 are excellent (S1 class) and the water is suitable for any crop and can be used for irrigation in almost all types of soils.

#SC Specific conductance (μS/cm).

###Although all these indices were evaluated in this study, the SAR is probably the only one in current use and is generally considered an effective evaluation index for most water used in irrigated agriculture.

Use SOLUTION_SPREAD for large water sample dataset


USER_PUNCH   #calculations use meq/l, except TH.  Convert mmol/L to meq/l = mmol/L X charge (abs)

-headings TH_(mol/L)   SAR   RSC   EC(uS/cm)   PI   KR   %Na+ 
10 TH = mol("Ca+2") + mol("Mg+2")        # Make sure L of water used in SOLUTION.

20 SAR = (mol("Na+")*1e3*1)/((((mol("Ca+2")*1e3*2) + (mol('Mg+2")*1e3*2))/2)^0.5)   
30 RSC = ((mol("HCO3-")*1e3*-1) + (mol("CO3-2")*1e3*-2))-((mol ("Ca+2")*1e3*2) + (mol("Mg+2")*1e30*2))
40 PI = ((mol("Na+")*1e3*1)+((mol("HCO3-")*1e3*-1)^0.5))/((mol("Ca+2")*1e3*2) + (mol("Mg+2")*1000*2) + (mol("Na+")*1e3*1))*100   

50 KR = (mol("Na+")*1e3*1)/((mol ("Ca+2")*1e3*2) + (mol("Mg+2")*1e3*2)) 

60 %Na+ = (((mol ("Na+")*1e3*1) + (mol("K+")*1e3*1))/((mol ("Ca+2")*1e3*2)+(mol("Mg+2")*1e3*2)+(mol ("Na+")*1e3*1)+(mol("K+")*1e3*1)))*100     



Graphical outputs:
United States Salinity Laboratory Staff (USSLS) plot used to classify groundwater in terms of degree of suitability for irrigation: a plot of EC vs. SAR

Wilcox diagrams used to classify groundwater in terms of degree of suitability for irrigation: a plot of EC vs. %Na+