"I'm breaking my back today," said Keith Mahoney as he wheeled nine cases of iced tea and fruit punch from a Coca Cola truck to a Coke machine bearing a handwritten sign that said: "OUT - because of new Fairfax law."

Mahoney and his route supervisor Bill Meador had replaced the Coke, Tab and orange in nine machines in Fairfax County with fruit punch and iced tea in throwaway cans. Since they are noncarbonated, they can still legally be sold in cans.

"It's just double work for us," said Mahoney.

Fairfax County's antithrowaway container ordinance took effect Sept. 1. Since then, the switchover from throwaway containers to returnable bottles and nonpop-top cans for all nonalcoholic carbonated beverage containers has brought some problems for road salesmen like Mahoney and Meador, bottlers, distributors and some thirsty consumers.

At Canada Dry, sales manager Larry Roney said he soon will have to lay off one salesman and perhaps some drivers because of the loss of soft of soft drink sales in the county. "I'm more confused today than I was yesterday," said a harassed Roney, who was realigning his delivery routes to spread the work around.

"It has been an extra burden," said a 36-year-old 7UP driver, Frank Robinson. "The empties are coming in like droves." Since Robinson doesn't have room for them when his truck is loaded with full bottles, he said he must make a second trip to some stores to pick them up.

Mahoney augments his base daily pay of $9.50 with a commission based on the number of cases he unloads. Since volume is down in Fairfax, his pay is also down.

Coca Cola operations manager C. M. Byrd said he was making up the difference for his men who work in Fairfax County, "but I don't know how long I can stand it," he added.

Beverage distributors say there is an increase in demand for soft drinks in jurisdictions bordering Fairfax - by as much as 50 per cent in some neighboring jurisdictions, according to Byrd.

Under the new law, nonreturnable cans and bottles may still be sold in the county if they bear a sticker saying, "Washington metropolitan area - 5 cents refund." Merchants must reimburse customers 5 cents when they present a stickered can or bottle.

The retailers and the supplier say they are not eager to place the stickers on the containers because of the time and cost. "What would we do with (the cans) when we got them back?" Byrd asked.

The empties "would make too much of a mess in the back room," said Jerry Heier, assistant manager of the Springfield A&P. Right now, his store is stickering only A&P own brand of beverages and for a temporary time, some Canada Dry.

The suppliers said they don't like the sticker option because they fear counterfeiting of the stickers, which would allow people ot get a refund on bottles that had not been purchased in the county.

In most cases, bottlers have chosen instead to pick up the cans they had delivered to merchants before the ordinance went into effect and to supply them with only the products that already were bottled in returnable containers. Canada Dry drivers collected 10,800 cans in eight days after the ordinace went into effect.

The suppliers, who have known about the ordinance since December 1975, say they don't like the idea of putting all their products in returnable bottles. Pepsi-Cola spokesman W. Edward Gregory said it would require a lot more trucks and "we don't see a return on our investment which would make us go into a total returnable line."

Canada Dry's Roney cited problems in obtaining the returnable bottles at the present time because mold-makers in the glass industry are on strike. Coca-Cola's Byrd recalled that his company abandoned a three-year campaign during 1971-1973 to get people to buy returnable bottles because it failed. The total use of returnable bottles "is just not feasible, the attitudes of people won't allow it," he said.

Despite bottlers' complaints' the county government said it has had "just a few" complaints from consumers about the inconveniences brought on by the new ordinance. The law's sponsor, Supervisor Alan Magazine (D-Mason), said the people he meets are "overwhelmingly in favor of what we're trying to do. I'm certain that within a short period there will be non problems whatsoever."

Magazine said he hopes that this reluctance to convert the vending machines was not an "attempt on the part of the beverage industry to sway public opinion," adding that he thought such a tactic would not work.

"Frankly, the citizens have adjusted to this change much better than the industry has," Magazine said.

For now, there is some exasperation, especially among office workers who must drink iced tea or fruit punch instead of their favorite soda. Distributors say they don't want to convert vending machines so that they can dispense bottles because of the expense and the fear that consumers will not return the bottles. The average cost of a returnable bottle is 17 cents, Byrd said.

County police report there has been only one known violation of the ordinance so far. An ice cream vendor was given a citation for selling a pop-top can of cola. He could be fined up to $500 and given a six-month jail sentence.