The first reason for this is water adsorbs lead mostly when the water is standing still, not flowing and how Rome did its aqueducts and pipes the water ALWAYS was flowing (the various fountains in Rome in Ancient times and restarted during the Renaissance is the result of such constant flow of water.
The second reason is the water in the aqueducts tended to have high mineral content, which tended to line the pipe with they residue, isolating the lead from the water in the process. This lining was about a mm per year about an 4 inches in a century.
On the other hand lead was used in producing "wine" (Rome had a bad problem of Counterfeit wine, in fact one ancient writer said the cheaper the wine, the better it is for it is more pure and NOT counterfeited with derivatives to make it taste better, as was the case with more expensive wines). This "improvements" on the wine included the use of lead pots, which could in theory lead to a massive increase in lead consumption.
The problems are the numbers needed to achieve such insanity, are way higher then the numbers needed to produce other problems:
Although the relationship between ingested lead and blood lead levels varies, and there is no way to quantify what Romans actually ate or drank, he further estimates the total amount of lead absorbed in the blood from all sources to be 50 micrograms per deciliter (µg/dL) or 0.5 mg/L. Three deciliters of such wine, the equivalent of two glasses, would therefore contain 150 µg of lead. Wine poured from Columella's amphora would contain 21,000 µg/L or 2,100 µg/dL.
Such levels have significant physiological consequences. As reported by the U.S. Department of Health and Human Services, there is brain and kidney damage in adults with blood lead levels of 100 µg/dL; gastrointestinal symptoms such as colic, with levels of approximately 60 µg/dL; anemia with levels of 50 to 80 µg/dL; neurological symptoms with levels of 40 to 60 µg/dL; depressed sperm count with levels of 40 to 50 µg/dL; and increased risk of preterm delivery, low birth weight, and impaired mental development with maternal blood lead levels of 10 to 15 µg/dL. The physiological insult to children would have been even greater. In children five years or younger, the Centers for Disease Control and Prevention recommends intervention when the level of lead in the blood is above even 5 µg/dL.
http://penelope.uchicago.edu/~grout/encyclopaedia_romana/wine/leadpoisoning.htmlThus, if the source of lead was from the wine itself, you are having 21 times what the US Department of Agriculture reports when you will start seeing solid signs of Lead poisoning. People would notice and someone would have stopped the poisoning IF THE LEAD WAS THAT HIGH.
Caligula had problems, he ruled for less then four years. He was born in 12 AD, but his Father was poisoned in 19 Ad, (it is believed on orders from Tiberius). Thus at the age of 7 Caligula was being involved in the open fight between his mother and Tiberius that was a continuation of the fight between his father and Tiberius.
Caligula's father, Germanicus, had defeated the Germans after the Germans had destroyed three Roman Legions when Augustus tried to conquer Germany. Tiberius always viewed Germanacus as a threat for he was popular with the troops given his victories over the Germans. Germanicus's widow, Antonia, was as popular, as where his children. Antonia was the daughter of Octavia minor, the sister of Augustus.
http://en.wikipedia.org/wiki/Germanicus
Now, you must remember Mark Anthony's first wife. She is forgotten in all the movies about him, for she was in charge of Italy after Julius Caesar's death and made sure Mark Antony's claims were protected even if that meant use of troops while Mark Anthony was away in the East. In her efforts to protect Mark Anthony's interest she ended up fighting and dying, but it was her efforts that made Mark Anthony the second man in Rome behind Augustus. This history was not forgotten, for at her death Mark Anthony married the sister of Augustus and together their produced Germanicus's father.
More on Mark Anthony's first wife, Fulvia
http://en.wikipedia.org/wiki/Fulvia
On Mark Anthony's Second wife and Augustus's sister (She ended up taking care of all of Mark Anthony's children, not only her own but Fluvia's and Cleopatra's.
http://en.wikipedia.org/wiki/Octavia_the_Younger
Octavia was much favored by her brother, she was six years older them him and in their younger years had watched over him (almost a mother-child relationship then a sister-brother relationship, thus Anthony's divorce of her was unforgivable by Augustus).
Octavia and Mark Anthony did have children, one of who, Antonia the elder (also known as Antonia Minor, Minor being the latin version of "Younger", became the Mother of Germanicus. Thus Germanicus was the grandson of Mark Anthony and the grand nephew of Augustus. Thus Germanicus was the grandson of Augustus's favorite sister. This gave him an edge over Tiberius in any fight over the throne for Augustus had no sons except by adoption (and these adoptive children stayed with their natural parents, the adoption was stickily political)
http://en.wikipedia.org/wiki/Antonia_Minor
Thus Germanicus was a heir not only to Augustus (via Germanicus's mother and grandmother) but of Mark Anthony who was his Grandfather via Antonia. Thus Germanicus was popular not only with the troops but the people of Rome. Under Tiberius that combination was a death sentence.
Thus Germnaicus was poisoned.
Now Tiberius has one son, Drusus, but under the Plot Praetorian Guard Sejanus he was killed by his own wife (or died of heavy drinking, the record is unclear). Drusus had one son, but even Tiberius believe that son was really the son of Sejanus NOT Drusus. That grandson of Tiberius, Gemelus, was as first was adopted and thus the heir of Caligula when Caligula became emperor, but later that same year Caligula had Gemelus killed:
http://en.wikipedia.org/wiki/Drusus_Julius_Caesar
http://en.wikipedia.org/wiki/Tiberius_Gemellus
Caligula had two older brothers, both were killed under Tiberius for alleged plots against Tiberius. These appear to be the fears of a mad man, but Tiberius by then was NOT living in Rome but on Capri so lead was NOT a factor.
Thus by the time we get to Tiberius's death, he had no sons, and his only living relatives where quite young, Caligula (age 25 when he became Emperor) and Gemelus (age 19 at the time he is executed by Caligula).
Caligula had seen his father killed, his two older brother killed. his own mother was arrested, exiled, flogged (in one flogging she lost an eye) seeing her situation as bad, she starved herself to death (Hunger strike till she was permitted to go back to Rome).
More on Caligula's Mother Agrippa the elder:
http://en.wikipedia.org/wiki/Agrippina_the_Elder#Time_in_Rome.2C_downfall_and_posthumous_honors
Given the above treatment of his Father (dead when Caligula was 7), Mother (dead when he was 21), and brothers (both Dead before Caligula was 20, recorded as have been starved to death) and his uncles and cousins (all dead, most by unnatural causes). Caligula had to be careful, then he gained all power as Emperor. He was careful at first, but when he found out that the Army and the People of Rome wanted him for he was the son of Germanicus it went to his head.
One thing to note, at that time period it was required of all troops and Roman Citizens to know how to swim, yet Caligula could not. This supports the theory he had Epileptic fits for people with Epilepsy are discouraged from swimming in case they had a fit while swimming.
My position on Caligula is he shows all of the aspects of a Spoiled Brat AND someone who is recovering from post traumatic shock (The death of his brothers and Mother more then the death of his father, for the former were when he was in his late teens or early 20s, the death of his father was at age 7, an age young enough for him to recover from such a loss quickly). His demand to be treated as a living god even in Rome shows a NEED to be in demand, a reflection of his years in isolation when his mother fought for his and his brother's rights after the death of her husband.
Now, most of the talk of his insanity appears to be post assassination justification. i.e. he had to die for he was insane. On the other hand his plan to move to Alexandria and abandoned Rome, put both the Senate and Praetorian guards on edge. i.e. if he left, he was the commander of every Legion, the Legions were his personal army not the Army of Rome. The bureaucracy set up by Augustus was to pay for that Army, This Bureaucracy was independent of the state for it was considered part of the household of the Emperor, and thus by the time of Caligula was more powerful then the Senate.
Thus the movement of Emperor to Alexandria meant the Emperor, the Roman Army and the whole government to support that army would go with it.
The only organization that could have STOPPED this move would have been the Plebeian Council, but technically it had turned over all its power to the Emperor under Augustus, but still meet till the last years of Tiberius's Rule. The Council could have been the means to get the people of Rome to object to the move in mass, but it no longer meet (but the memory of it still survived).
http://en.wikipedia.org/wiki/Plebeian_Council#From_471_to_27_BC
Furthermore the Senate of Caligula was no longer the exclusive domain of the old aristocratic families of Rome. Most could not trace themselves further back then to Julius Caesar. The old families were still around and connected to the Senate, but no where near the power they had just 100 years before. In the last years of Tiberius's rule, these two groups had effectively merged and had ruled Rome quite well during the time Tiberius was in self imposed Exile. Caligula bringing the Emperor ship back to Rome changed that and a series of conflicts arouse. When the conflicts became so bad that Caligula decided to move to Alexandria, it was decided he had to go. The Senate was NOT looking to find a new Emperor but to continue what they had done in the last years of Tiberius's rule. To do that Caligula had to go and he went.
The problem was the People of Rome still like Germanicus's name and when Caligula's uncle (Germanicus's brother) was found they made him Emperor. There was some protests, but the new Emperor could work with the Senate and together they ruled Rome till he died and Nero became Emperor.
Thus the whole attack on Caligula may just be justification for his assassination and the attempted coup against him. Claudius did not care to defend his dead nephew for it did not affect his rule, so the attacks on Caligula were permitted and became worse over time. This does not mean Caligula was a Good Emperor, but they is no real evidence he was insane or a truly bad emperor, but one that could NOT work with the Senate as his Uncle was able to do.
= new reply since forum marked as read
the burning tears of sarcasm 
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would never apply to a conservative
