Potential Impacts of Water Quality &

Chemistry on Herbicide Applications

Dan Campbell

Exotic Plant Management Specialist

National Park Service

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Primary Water Quality Issues

pH: acidic, neutral, basic

Turbidity: suspended solids & organic matter in

Dissolved mineral content: charges particles (ions)

Your Water Can Change

• Hardness and pH can change during the spray season depending on your water source.

• Is your water source from a municipal source or from a well?

• Municipal water doesn’t mean that you have good water.

They don’t adjust pH or hardness.

• As well depths and aquifers change so does the water quality.

• Water should be checked.

• At start of spray season

• Mid summer

• Late fall

pH

Water (H2O) breaks down into ions of H+ and OH-

pH = The negative logarithm of the hydrogen ion concentration [H+]

(as H+ increases, so does acidity)

pH 1-6 = acidic, pH 7 = neutral, pH 8-14 basic

Many herbicides are weak acids (glyphosate). These release H+ ions

and dissociate in water.

pH

Weak acids dissociate less as the solution pH decreases – more

active ingredient available.

Generally, the ideal pH for water used to apply spray materials is

slightly acidic (pH 5 – 6.5). Some exceptions exist. Read labels.

Movement of Ionizable (Weak-Acid)

Herbicides

Weak-Acid herbicides in an acid environment are not ionized and

can freely cross plant membranes, upon entering the alkaline

phloem (high pH) they will become ionized. This “traps” the

pesticide in the phloem and will subsequently be transported to

“active sites” within the plant.

Non-ionized pesticides freely

cross plant membranes

pH 5 Xylem 150 cm / hr

Phloem – 90 cm / hr pH 8

Roots

Leaves

Non-ionized pesticides freely

cross plant membranes

Solubility vs Spray Water pH

• As the spray water pH changes, some products increase in

solubility.

• As solubility increases, it increases the bioavailability of active

ingredient for better and more consistent efficacy.

• Glyphosate, triclopyr amine, imazapyr, & aminopyralid are very

soluble – effect of pH negligible.

- aminopyralid -

Turbidity

Koc: organic carbon sorption coefficient. The higher the value, the

more likely the active ingredient will adsorb to organic matter.

glyphosate = 24,000

aminopyralid = 2,500

triclopyr ester = 780

imazapyr = 100

triclopyr amine = 20

Particulate texture: clay, silt, sand

clay – greater surface area; more likely to

possess an ionic charge.

Dissolved Mineral Content

Water is a “universal solvent”

TDS – total dissolved solids

Hardness is expressed as the amount of calcium plus magnesium

present as calcium carbonate equivalent. It can be expressed as

parts per million (ppm) or grains per US gallon.

(1 ppm = 17.1 grains/1 US gal)

How is TDS measured?

• Weight

• EC

• Chemical reaction

(test strips)

Calcium = 666 ppm

Sulfate = 2434 ppm

Magnesium = 234 ppm

Chloride = 32 ppm

Sodium = 130 ppm

Bicarbonate = 346 ppm

TDS > 500 mg/L (ppm) is salty to taste

3842 ppm

ppm/Gal Grains/Gal

Soft 0 - 60 0 – 3.5

Mod. Hard 60 - 120 3.5 – 7.0

Hard 120 - 180 7.0 – 10.5

Very Hard Over 180 Over 10.5

Reference: Montana Environmental Health Association – Sanitarian’s Field

Guide

What is hard water?

Drinking water <500 TDS (EPA)

Fresh Water : <1,000 TDS

Brackish : 1,000-5,000 TDS

Highly Brackish : 5,000-15,000 TDS

Saline : 15,000-30,000 TDS

Sea Water : 30,000-40,000 TDS

Brine : 40,000-300,000+ TDS

Types of Water

Common Mineral Elements in Water

Cations (+) Anions (-)

Calcium (Ca++) Sulfate (SO4-)

Magnesium (Mg++) Chloride (Cl-)

Sodium (Na+) Bicarbonate (HCO3-)

nitrate (NO3-)potassium (K+) iron (Fe++, Fe+++)

Water (H2O) breaks down into ions (charged particles) of H+

and OH-. pH is an indication of the H+ ion concentration.

What is pH?

Dissolved minerals specifically interfere with the performance of

some active ingredients such as:

2,4-D, aminopyralid, clopyralid, glyphosate, imazapic, imazapyr,

triclopyr amine.

2,4-D amine is completely “deactivated” at 500 ppm water hardness.

Glyphosate noticeably less effective at 150 ppm hardness.

Mineral Antagonism

What can you do?

Reduce your water volumes

(20 GPA ties up 2X the glyphosate as 10 GPA)

Use maximum application rates

Use a water conditioner to block mineral antagonism and enhance

uptake.

Water Hardness Test Strips

What can you do?

www.herbicide-adjuvants.com

Additives to improve performance

Adjuvants

Conditioners

Ammonium Sulfate (AMS)

Non-ionic surfactants (NIS)

pH

Hard

Water

Acidifiers

Buffers

+Ca

-Glyphosate

Addition of Ammonium Sulfate…Why?

2NH4 SO4

- -++

The sulfate ion “blocks” the mineral cation.

The ammonium ions attach and “escort” the glyphosate into the plant.

-

++ - -

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QUESTIONS?

Remember

To maximize the performance of glyphosate the spray

water pH needs to below a pH of 6.0

Glyphosate is noticeably less effective at as low as 150

ppm of water hardness.